for my own opinion

hordes:
shaman = tauren
warrior = tauren or orc
hunter = orc or troll
priest = undead or troll
rogue = commonly undead
mage = commonly undead

the rest i duno, as for alliance

alliance:
warrior = human/dwaft
druid = elf only i think
warlock = gnome/human
mage = gnome
rogue = gnome/human
palladin = dwaft/human
hunter = commonly elf = = but other class i still think is ok, but saw many dwafts for hunters


all this base on my experince and i wont define it as correct in terms of wow due to every ppl also have they opinion and belives

well i admit that UD warriors are great in pvp but tis dosent mean orc and taurens suxs in pvp tho....and the 25% resist stun really helps alot for the orc and taurens war stomp owns in any pvp ground...just play which u think is nice for ya