Well, obvious forum perks include being able to colour your name, set your own rank etc.

Access to hidden forums is another obvious one.

I've seen examples of the username coloring, but that's how you differentiate between user levels (member, moderator, admin, etc) here. User titles I'd definitely like to add, and secret communities may (or may not..) already exist.

You're talking to yourself with two different accounts in a secret forum, aren't you alex?

I already assumed he was doing that a while ago.
Maybe take out the ability to edit your signature as much.
Gaining the ability to add coding to your signature, such as bold and italics, as you level up. Very small, but distinguishes the higher ranks.
Obviously a level under the name on posts.

Oh! and somewheres to see what you'll be gaining on the next level, and maybe what you've already gained?

pssst. editing too. haha. =)

Please keep these ideas coming, I need moaarrr! I am going to start getting the karma system fully integrated.

Mitch, I'm sure you've noticed that I finally added post editing. :D It is super convenient.