Most of the rewards are extra costume pieces for all your characters (trenchcoats, wings, that kinda stuff) or something special, such as a unique weapon, or, if you've been a member for all thirty months total, a free little mini pet that'll follow you around, like the World of Warcraft collector's edition pets. It's not cumulative, so if you unsubscribe like I did and come back, you still get credit for all the months you've ever played.
Me, I got up to the level where you earn a set of samurai armor pieces. Putting two and seven together, I reached into my bag of tricks and pulled out
PS. You can't make a good John Belushi samurai with the hero creator. I've tried it. You can't get his topknot right. Plus, he just wears robes.