General

credit goes to @Traitor - his words verbatim. don't question the integrity of it.
All of this research was done on his own.

December 2010 - Big Bang: This is the biggest noticeable change in content.
Pretty much all all you know and loved has been altered. The user interface has changed, map layouts and monsters are now changed and adjusted for user friendliness.
Release of the "Resistance" branch of classes in the city of Edelstein, which has fallen under control of the Black Wings, who work under the black mage.

Three new classes are of this branch:
Wild Hunter - Crossbowman who rides a jaguar [first mounted class]
Battle Mage - Melee mage who uses staves
Mechanic - a mech-suit gun user

June 2011 - Chaos:
Arans & Evans got their skills buffed.
This is the release of the Profession system which replaces the old Maker system and completely overhauled crafting.
Now you can learn:
Alchemy to brew potions.
Jewel Crafting to make accessories.
Smithing to make weapons & armor.
You can currently chose 2 different types.

New Boss: Cygnus [Empress]
New Empress [Cygnus] boss has been released, dropping 140 equipment for each class with set bonuses.

September/October 2011 - Ascension:
PQ revamp, adjustments were made to PQs and new PQs were released.
The three primary explorer classes [Warrior, Mage, Archer] received a buff.

November/December 2011 - Legends:
Release of Cannoneer the additional Pirate Adventurer class which has a different storyline and Mercedes a new Legend class that uses Dual Blowguns which goes by an elf storyline, Demon Slayer a new Resistance warrior who uses 1h Mace/Axe which uses the new Demon Fury system instead of MP.

June/July 2012 - Renegades:
Release of Phantom, The Thief Legend class
He has the ability to steal one skill from any of the explorer classes for each job advancement as well has having his own.
Explorer thieves & pirates skills received a small buff.

September 2015 - New Dawn:
Release of arguably the best class ever: Mihle. really op right now. so in the meta

November/December 2012 - Tempest:
Release of Luminous the mage Legend class along with the release of the Nova classes:
Kaiser - A dragonoid human who uses 2-handed swords, being able to transform using Dragon-like skills.
Angelic Buster - A dress-up character which also has the same wings as Kaiser having a transformation form. Angelic Busters can only be made as female being the first Gender-locked class. They were also the most OP at their prime due to being the first class to break the 999k cap. They also use a new soul charge system instead of mp.

April/May 2013 - Mark of Honor:
Release of Sengoku classes:
Hayato: a warrior samurai who uses the new weapons Katana
Kanna: a fan wielding mage class

May 2013 - Cygnus Returns:
Dawn Warrior/Wind Archer/Thunder Breaker completely revamped with completely new skills as well as their max level cap raised to 250 from 120.

June/July 2013 - Unleashed:
[i]-Damage cap raised to 50,000,000 from 999,999.
-Seen damage range cap increased to 2,000,000 from 1,000,000.
Level cap of all classes increased to 250 from the previous 200.[/i]
Bosses were also revamped to align with the new damage cap.
Release of Xenon a Resistance class based on cyborgs, also being the first hybrid class being both Pirate and Thief and Demon Avenger the second Resistance Warrior class to also following the demon theme using a newly developed HP system for their skills instead of MP.

New Bosses: Queen, Pierre, Von Bon, Vellum
Release of Root Abyss where the bosses drop lv150 set armor/weapons which drop from the Chaos version. These equips were a new set that made Empress obsolete.

November/December 2013 - RED:
Explorer classes got revamped to balance out the newer classes receiving their necessary buffs.
Release of the Zero class, the first tag-team 2h-sword warrior class where you control 2 characters in one. Also the first class to be created starting at level 100.

June/July 2014 - Rising Heroes
Release of Shade, the forgotten Pirate Legend using animal spirits as an aid to their fists.

August/September 2014
The continuation of the Cygnus revamp finishing with Blaze Wizard and Night Walker revamp. The skills have been changed and the max level has been changed to 250 from 120.

November 2014 - Star Force
Equipment enhancement scrolls are now obsolete with the new Star Force system allowing you to reach stars on regular equipment up to mainly 12 stars prior to booming. [Low level equipment has a maximum of lower stars] Special equipment such as tyrants boom much earlier. This system also has the chance to drop in the amount of stars.
Release of Star Planet, a smaller world where you can play the event mini-games such as one-card all the time for special prizes and bonuses.
Reward Points were introduced in this patch, these points are acquired by killing bosses and special daily tasks as well as reach certain levels. These are used in the cash shop for rewards shop prizes as well as being able to use them as a 30% discount on most items if they are eligible.

April/May 2015 - Black Heaven
New Boss: Lotus
A new storyline that unlocked the currently new hardest Boss Lotus, the brother of Orchid a member of the Black Wings. This also has another set of equipment arguably better than the CRA set but has not been yet proven, though it's said CRA set is still better.

In between Nexon has been doing more events such as more frequent 2x, Spell Trace fever[additional success rate chances to scroll items with Spell Trace], Miracle Time[additional chances to raise potential tier when cubing with cash shop cubes and double tier ups]. Most maps at this point have been revamped and everything you know and have come to love pre-bigbang is no more. Monster levels and HP have been adjusted to the new level ranges as well as damage ranges and party-play in maps such as Lionheart Castle, Strong Hold, Hall of Honor are no more.
-----------------------------------------------------------------------------------------------------
I'll send you the guide that I wrote for someone else a little while ago. I hope it helps.

If you need help in any of the steps below, feel free to PM me. [Note: Most things in this guide does not require NX, you just need patience]

Training, most people grind lower levels til you can PQ and PQ pretty much levels you fairly quickly to the major grinding phases at ~160. FM seems a little more empty now but there are a lot more shops around, especially in Scania. Since owls are now purchased through pot shops, you can easy pick some up and just search for the items you need[other that Perm NX items].

Basic training guide:
-Starting from level 1-10, you pretty much just job advance as usual.
-Once you're level 10, you can go to Golem Temple in Henesys in which you can level there until 25-30.
-Once you're level 25-30, you can move to Gold Beach. I would start on the left side map with the coconuts and work my way up to Shockwave Beach 1. I would stay here until around maybe level 50-55 or so.
-Starting at level 50, you can go LudiPQ. PQs are now all gathered in one place available by the Dimension Door in most towns. I would Ludi PQ until level 60. [If you do not wish to PQ the whole way, you can go to drakes at around level 55-60 and work your way to 70]
-Once you are 70, I would do Romeo and Juliet PQ and stop and grind at the stage with the Roids. This stage is the best for exp even grinding outside of PQ best optimized with a Phantom/Priest with HS and a Kanna for Kishin. Other Alternatives are Sahel 2 in Ariant. You can stay grinding in this PQ until 140-150, it's pretty much the fastest way to level IMO, unless you wanna try to grind in some maps in Magatia which is a bit slower.
-Once you hit 140 you can do a party for Dimension Invasion PQ if you can't solo it yourself, you can pretty much do your daily 10 PQs and then go grind at Mysterious Path 3 in Singapore. Once you reach 165, you can now enter Stronghold. Note:Honestly, I really don't recommend it unless you have friends to leech off of or unless you can do 1-3 hits. Otherwise, the exp isn't as great and you're better off doing your daily DIPQs. You can pretty much farm the gloves in DIPQ and sell in a shop for a couple mil apiece, I'm not sure where the price stands right now.

You can boss in-between for optimal experience. The default key is O in-game I believe for the boss tab so you can check the requirements there. [You can NPC the drops and make a couple mil a day, more bosses, more money. Start with the easy and work your way up. Most bosses have 2 entry limits]

Items to think about having eventually for end game: (This may prove to be expensive, but I will post the affordable equips after]
-Chaos Root Abyss Set [Hat, Top, Bottom, Weapon]
-Tyrant [Glove, Shoes, Cape, Belt]
-Sweet Water [Glasses(Eye), Tattoo(Face)]
-Superior/Reinforced Gollux Set [Ring, Pendant, Belt, Earring] (if you use Tyrant Belt, you would have to use NX to purchase a pendant slot to equip 2 pendants to get the full set bonus]
-Rings [Since the Gollux Set takes up a ring slot and unique equips, you can pretty much get one of each Gollux RIng, if not Lightning God Ring for the best stats]
Note: This is the generally ideal set but by all means you do not need to chase this as this is VERY expensive.

Cheap but Efficient Equipment Guide:
The CRA set is a couple hundred mil apiece but there may be some pieces out there depending on class for a couple mil. If you cannot get this, you can try to buy the empress set for the bonus effect stats. Personally, I would say save your money and at around 140, you can grind at the higher leveled monsters in Leafre or the Pirates in Herb town. They drop the new level 130-140 set which gives a set bonus as well. If you take this process, you can save your money but slowly increase your damage. These level 130-140 sets are also tradeable so you don't need to purchase use SoK or pSoK to transfer them character to character. Next I would try to purchase cheap solid/reinforced gollux pendant/belt for small range increase. If you hunt bosses, easy bosses around be Zak/Horntail/Hilla/Magnus/Ranmaru in the easy/regular modes(you can do these twice daily, bring a friend if you must), I wouldn't attempt harder modes until you're geared and much stronger. The bosses drop accessories[Eye,Face,Ring,Earring,Badge, etc.] that also have set equips. If you find some Epic / Unique accessories, you may make a quick buck. Remember, though you won't be getting rich fast but patience is the key.

Scrolling:
-The scrolling system has been semi-replaced with the Spell Trace system which you collect a required amount of traces to use a scroll. How it works is you double click the Spell Trace in your etc tab or the Pink Hammer in your inventory window and place the item in there. You can then see how many traces is required per scroll. If you're poor, I recommend 70-100%ing your items, except for weapons which I always recommend at the very least 70%ing it.
-The enhancement system is semi-replaced by the Star Force system, this system takes effect after you finish scrolling an item. Once the item does not have any slots left, you can add additional stats by enhancing it with Star Force which is done by the Pink Hammer. I recommend Star Forcing all your equips until 10 stars because it is fairly cheap to do so. This will also increase your range by a pretty fair amount.

Potentials:
There a certain tiers for items to have potential which has 4 different ranks. I wouldn't really worry about potential items unless they are above level 75 which is the 3rd tier.

First Tier: 1-30, items will generally have lines in multiples of 1%. (2%, 3%, 4%)
Second Tier: 31-74, items will generally have lines in multiples of 2%. (4%,6%,9%)
Third Tier: 75-159, items will generally have lines in multiples of 3%. (6%,9%,12%)
Fourth Tier: 160+, this one is a little unique because the lines are 4% (7%, 10%, 13%)

How it works:
if your item is Rare, you will generally have the lowest multiple.
if your item is Epic, you will get the maximum landing at 2nd multiple.
if your item is Unique you will get the maximum landing at 3rd multiple.
if your item is Legendary you will get the maximum landing at highest multiple.

So for example, your Legendary item unlocks the potential to have the maximum % but you can land any of the %s below it, such as rare, epic, unique. So if you get VERY lucky, your lines would all be of one stat but don't be so sure to expect to get that easily. You will be spending many, many, packs of cubes. The best time to cube is during miracle time when nexon does a 2x-like event for cubing, increasing the chances of rank up. Again, I don't recommend cubing anything below the third tier.

Weapons and armors have different potentials. Weapons have additional stats such as att/matt where as armor is generally stat based. You can make cubes but farming the materials and leveling the skill level takes time. I would recommend picking up professions as soon as possible to start leveling it up. Sometimes when you make equipment, such as earrings, you can get epic/unique items which you can sell for a fair amount, meaning you can make money this way. When you mine mysterious rocks you also have a chance to find cubes(other way would be bossing)!

Nebulites:
You don't really get to see this much but people generally applies this to weapons. Select few rich players have acquired a lot of nebs for their armor as well.

How it works:
You can purchase a nebulite socket opener thing from the pot shop and apply it to your item. You can then apply your nebulite of choice depending what you have.

Weapon: I would put on a %boss or %pdr neb or % att neb for endgame. If you can't afford it, you can use an att neb or crit neb. Think wisely because removing a neb costs NX.
Armor: I always recommend %stat nebs but if you're poor, again you may choose to use simple +stat nebs

There are some useful but expensive skill nebs with skills such as Hyper Body for pants and Sharp Eyes for gloves meaning that regardless of class you can use both those skills but at a nerfed rate. The stats only apply to yourself however.

Souls for Weapons:
This is a newer system where you can even more stats to your weapons.

How it works:
You use a soul enchanter on your weapon, these drop from monsters and some bosses. Once your item is a soul weapon, you equip your weapon and apply the soul. If you do not have a soul, you would have to hunt bosses which drop soul shards. Collect 10 shards to trade for a random stat soul in which you can apply to your weapon. Some souls are attacks, some are summons.

@Traitor