UPDATE: The magic number is now 20! Read our updated version of mounts for the leveling player.
In the World of Warcraft, 40 is a magic number. Once you hit this level you get access to a 31 point talent, which usually does some pretty nifty things for your class. You really start specializing into your role as a healer, tank or DPS character. The best part about 40 is getting your fancy new mount. Once you get used to traveling the world on you pony (wolf, nightsaber, raptor, ram, etc) it can be really hard to go back to hoofing it on foot.