Some races look more like eachother than others. I found that the Tauren all look quite the same. Probably because they're so big, you don't pay much attention to the face/horns/skin color...
The undead for example are very diverse. You can give them crazy hairstyles, different face features (no lower jaw, no eyes,...).
All the other races are pretty diverse IMO. You'll have to find clothes in-game, which might be hard.
If you want voice options, you should probably run another program like Jonbo said. But you'll need to add friends first..
Blizzard decided not to implement that because it takes up huge bandwith. And probably more reasons.
You'll enjoy this game.
