Well there's a few things that make this game not so much fun for me anymore, hence I stopped playing:
1) Too much WoW influence, you can see it in the level cap, style of the quests etc.
2) Storyline was pretty bland and predictable.
3) Preferred D2's dark graphical style.
4) I know Diablo can be repetitive, but I actually found myself missing things like Pindle runs.
5) Pricing items on the AH was a proper pain in the ass. Endlessly waste time putting in filters just to find out what something is worth.
6) The chance of getting iLvl 63 items is low enough, without the chance of that item actually having a decent roll on top. I just gor frustrated id'ing a whole page in my stash of iLvl 63 items and finding not one of them worth a damn.
7) Miss being able to jump between acts at will.
8) Uninteresting legendary items, mostly vendor trash or sell for the price of a brimstone.
9) I played for approx 250 hours across my Barb and my DH, and did not once find a single set item. Bad luck, or awful drop rates?
10) To go from decent gear to better gear can most millions upon millions.
11) Finding upgrades for yourself once you have decent gear, almost impossible! I didn't in the whole time I played, had to go to the AH.
Things that D3 does better than D2? There are a few things:
1) Automatic Gold pickup, life saver. Just wish they made the radius bigger as you got to Lvl 60 instead of making it it an item roll.
2) Better inventory management, smaller items etc.
3) Ability to use multiple specs without respeccing.