What I loved in 3E:
1) OGL and SRD. Finally a license that does two important things: 1) clarifies and clearly defines what is allowed and what is not; and 2) allows you to create fan (or even commercial) material for d20 within these guidelines without having to deal with any significant copyright/licensing issues. This led to an explosion of new creative material, both fan and commercial.
2) Anyone could be anything; no level limits, no race-class restrictions.
3) Bonus spells for most spellcasters (2E had only for clerics).
4) The ability to add class levels to most monsters so if you want to fight, say, orcs at level 20 you CAN do it without it being a cakewalk.
5) Easy multiclassing (except for the math at higher levels, that is) without 2E's many restrictions on this.
6) AC that goes UP! No THAC0!
7) A unified core mechanic rather than having a different system for each portion of the rules.
8) A wide selection of options - you could find sourcebooks for almost any conceivable campaign type.
What I didn't like in 3E:
1) Prep time, prep time and more prep-time. This was the big fun-killer for me: it made running a game incompatible with my sometimes-tight RL schedule. It also made prepping a game into a chore rather than an enjoyable task.
2) Too many options, some of them having pitfalls; the need for rules mastery and optimized characters to avoid character-building pitfalls; building a higher-level character became a chore.
3) Too much of everything. This was both a blessing and a curse: on one hand it added a lot of variety, but on the other hand it meant too many skills, too many stats on the character sheet, too many stats per monster, and too many things to consider during combat (especially with higher-level characters).
4) Page-flipping and book-referencing during gameplay. This slows down the game and interrupts the fun.