Mounts: They get your ass from point A to point B, and if you're lucky, they'll do it as stylishly as possible. They're the most immediately visible status symbol in WoW and often give you a one-glance note on the kind of player at whom you're looking. People astride Invincible can be counted upon to be hardcore raiders from the Wrath of the Lich King era, someone riding a Violet Proto-Drake is never new to the game, and players rocking a White Stallion are seriously old school.
Players who have been around for any length of time almost inevitably accrue a stable's worth of beasties to cart them around, but some folks go the extra mile for the Leading the Cavalry and Mountain o' Mounts achievements. In Cataclysm, these are a lot easier than they once were, although you should still be prepared to dump a ton of gold and time into them no matter what. Fortunately, the first part of the Mountain o' Mounts trek should also net you The Ambassador achievement and title.