Here's the big things:
- -Loot now mostly creates relevant items to the character you are presently playing, and the rolls are less bullshit. (Anywhere from 1-300 agility is now instead 200-300, for example.) There are also many, many new legendaries with fun effects that aren't exactly game breaking, just vastly game changing. The downside is that they are Bind of Pickup UNLESS you're grouped with friends, in which case you can hand it off to anyone who was in the game with you when you found it for 2 hours.
-Difficulty is now changeable at any time. Yes, this means the legacy system of Diablo difficulty is gone (Normal, Nightmare, Hell, Inferno) but the end result is something that is fun and rewarding. You're playing a more up to date game with adjustable difficulty that gives better rewards. Higher difficulty has enemies with more health and damage, but you get straight up better loot and more experience. As someone who dealt with Inferno at release, I have to say the whole deal scales way better. The progression feels natural and doable rather than a steep fucking cliff they throw you off of after hell.
The class changes are good, but I feel like those two changes did a lot to fix the game. I wouldn't suggest reading over the patch notes, because those are insanely long.
Oh by the way the auction house is finally going away forever when RoS actually drops.