Good question. Of course the primary reason a lot of players choose their race is personal preference. Some races are just plain cooler.
Most of the more hardcore gamers look closely at the racial traits and skills possessed by each race when choosing what they will be. Each race on WoW has different abilities they are born with, many of which make them ideal for certain classes. The WoW homepage has a good listing of these racials along with profiles on the races. Be sure to check them all out before choosing your character's race.
Alliance:
Humans
Dwarves
Gnomes
Night Elves
Draenei
Horde:
Orcs
Trolls
Undead
Tauren
Blood Elves