Undead warrior here. At first, you will notice slight disadvantage compared to Orcs or Taurens. You start with less strength and stamina. However at higher levels 3 points in strenght would not make that much of a difference.
In PvP UD warrior is bad news, he can not be feared or polymorphed and, trust me, those are BIG advantages. You will have to learn to deal with pallies however - either run away if you 1v1 or let a mage rip a paladin apart if you're in a group (they are biggest concern for a UD warrior). Priests can be also a pain for UD, because of shackle undead. However, from my experience that is not that big of a deal compared to fear and polymorph.
As for Night Elf vs Human warrior = at lvl60 same thing. No racial traits yet, so that might change after they implement those.
Only thing I would not advise, is going a gnome warrior. I can't see those in the bushes - so they are hard to kill, cause they can hide behind a leaf on the ground.