Another thing that needs to be worked on in any future D3 is the system of leveling and gaining experience, IMO. It's silly how easy and quick it is to get a character up to level 50, or halfway to the maximum. People either play to go through the story and experience the quests, or they play in order to level a character and make a good build for dueling/mfing/bragging rights. I think they should make the first option more involving, with many more side quests (as was mentioned already), so many that it would take just as long to do all the side quests as to do the essential ones. But please, real SIDE quests, that you have to go out of your way to find/complete, not "oh hey, what luck, you'll pass right by the Horadric Malus on your way to Andariel, go grab it for me!" And make leveling slower from 1-50 and faster from 80-99. I can reach level 10 playing solo in five minutes, but in the same time with 8 players doing Baal runs, I can maybe get 1% of the way from 95 to 96. Something is wrong there.
Also, it would be GREAT to have variable rewards from quests! Right now, you kill Radament, you get +1 skill point. No matter how many times you die, or how quickly you kill him. Wouldn't it be great to have quests with many aspects to them; for example, in Act 5, if you had a time frame in which to save the barbarians, and depending on how many you were able to actually save, you get a different set of runes in reward. Or here's another example with a D2 quest: once you reach Act 2, the Maggor Lair slowly expands as you play, so if you get there right away it's a relatively small dungeon, but if it takes you a long time to get there, the maggots have dug longer tunnels and perhaps extra levels with more enemies.
They could even introduce conflicting quests, for instance, Andariel's presence is poisoning the Rogue Encampment (deja vu, anyone?), and if you manage to kill her before any rogues die, you get an extra reward, but you won't have time to do side quests or get extra levels so you'll be weaker and it will be harder to kill her, and maybe there's one side quest that you can't possibly finish and still do Andy quick enough for the best reward so you have to choose one or the other.
EDIT: Regarding leveling near the end, they need to add areas with higher-level monsters than the final boss of the game, and do away with boss running for exp! It's so boring! :yawn: