Here are some of the biggest differences that I was hung up on when first learning 3rd edition:
1) tactical movement on a grid
2) attacks of opportunity (for nearly everything)
3) feats
4) class "balance"
5) Challenge Rating
6) 0-level spells, cantrips, and ever-present spells
7) prestige classes
8) the d20 DC system for skills (that took away all DM rulings, as everything was codified)
9) character wealth by level baked into the system
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Anyone else realizing this?
It's definitely a generational shift, but it's not like all of those 9 things were brand spanking new with 3e.
1) Tactical movement on a grid was already part of the game considering it came from miniatures, had scales associated with miniatures, facing rules based on both square and hex grids, and grid combat in Player's Option: Combat and Tactics.
2) AoOs existed for the special case of fleeing a melee since at least 1e, PO: C&T expanded on them with threatened spaces and other triggers for AoO
3) feats = weapon proficiencies for the most part, and both OA and the Celts book introduced some things like martial arts maneuvers and Salmon Leaps that could be bought as special moves
4) class balance was just balance around a different metric as evidenced by different XPs for each class to advance (though it didn't work very well)
5) CR was a refinement of monster levels
6) cantrips were introduced in UA, though the implementation was a bit different
7) the 1e bard was essentially a prestige class
8) Yeah - this was a pretty significant change. Made them use, essentially, the same mechanic as hitting something in combat. It does make the game easier to learn, but WAY too much is made about it not allowing DM rulings. Player who were savvy with the rules were already lawyering the heck out of things if a DM let them. 3e just put more info out there in the players' hands by default.
9) Some elements of character wealth was already assumed by the system - wealth by level just made it visible and gave DMs a way to pace it (or deviate from the expected pacing if they so chose).
Basically, most of that stuff was a further development of ideas already percolating around TSR before they stopped being able to pay the printer. I'd even argue that none of those elements were the most far-reaching changes in how the game ends up being played - though 9 touches on it. Changes in magic items and their ease of creation/purchase blow all of those changes out of the water when it comes to the game being significantly different.