Level 20 is a fun milestone to reach. You usually receive a pretty powerful new ability for your class, but more importantly, you can also obtain your first riding mount! This riding skill, which at game launch was originally given to you at Level 40, lets you ride mounts that increase your speed by 60%. Every race has their own unique mount, and there are two classes that get special ones.
Gnomes get mechanical chickens.
Warlocks of all races get the felsteed, summoned from the depths of hell or something.
Paladins get the armored warhorse.