Agamon
Adventurer
* Concentration is such a big deal now. Buff spells, a huge part of 3.5E, now require concentration - and that concentration can be broken by taking damage, becoming incapacitated, or by casting another spell that requires concentration. And that includes readying spells or taking longer than 1 action to cast a spell!
* The spell slot/prepared spell system takes the sorcerer and wizard, mashes them together, and gives something even more versatile.
Both of these points are great changes. Buffs were one of my major gripes about 3e. Allowing them, but making them non-staking and interruptible is a great fix. And the spellcasting versatility is awesome. I agree that it seems so obvious now, but we needed to learn from the past to get where we are now.