Now that I thought about it, I can think of a few things that could've been done "better"
1. More content. Definitely. And allow the first person perspective for every class in the game. Oh, and more classes wouldn't hurt.
2. Harder enemies, more memorable bosses. As opposed to the ones that seem to be splattered easily. Just because you made Diablo II, doesn't mean you need to make the game mad easy like Diablo II is.
3. De-instance-ise the world. A game run solely on instances feels incredibly shallow, and doesn't help in suspension of disbelief. Which, in turn, makes the "storyline" rather shallow.
4. Fix bugs, particularily the "slingshot" bug that causes you to warp back to the spawn point at random intervals, without any lag being present.
5. Some method of "unsticking" yourself. Sort of like most MMO's have anymore. Or at least make it a lot harder to get stuck.It does happen, and a lot. Purely by accident, too.
6. More PVP. The game play, "story" and to a lesser extent the enviroment might bring people in. But why would they stay?