It's mostly the Tauren really. The Undead are repulsive (to me) and the only really "evil" race I've seen so far. The Orcs, much like the Humans, Night Elves, and Dwarves, have a been-there-done-that feel to them, and I really not too fond of their opening areas...too post-apocalyptic.
The Trolls are interesting, but it really came down to the Taurens for me.
They look great, and their culture/lore has a heavy Native American slant, which is awesome. Very spiritual, very in-tune with nature, very peaceful. Their opening areas are full of green and sunlight. The War Stomp is arguably the best racial ability in the game. And their capital city is one of the best looking. Plus they're the root race for the Shaman class.
Couldn't really deal with Night Elves or Humans. No offense, but Night Elf players are a dime a dozen. Everybody wants to be the Legolas pointy-eared type. I could do without the cliche. Is like Street Fighter, where everybody and dey grandmother playin Ken and Ryu.
The Horde in general has a great underdog feel to it. Other than the Undead, none of them are actually "evil" either.