But besides the game balance problems, the fact is that the racial abilities are about as well-represented as they're going to get (and in fact, if there are any changes in the future, they'll probably be towards conformity rather than radically away from it). You can't skin Tauren because, guess what, they're humanoids. Undead can't be immune because guess what, they're humanoids, too, and while a weakness to holy spells might make the game interesting, it won't help towards balance.
The Warcraft world is a mean one, but would the Alliance really go so far as to skin fallen Taurens on the battlefield anyway? For game balance or for lore reasons, it just doesn't make sense.