Collision-Detection?

Hey!
I’ve been thinking quite a lot about this recently.
Say i want to create a keyboard-controlled “wasd” game with proper hitdetection, doing this tilebased is no feat and is pretty straightforward.

But say i want to do this in the fashion for a game like WoW, and things quickly starts getting trickier. How would i resolve something like this with AS3? One possible solution would be to implement Box2D, but since I’m planning to make a rich 3d-game, the question is whether 2d collision-detection will be sufficient.

So I’m throwing out the question to you guys. What options are there for collision-detection in AS3 if the aim is to create a game with similar mechanics to WoW?