Mutants and Masterminds anyone? Or any superhero RPG?
MWP's Cortex-Plus-based Marvel Heroic Roleplaying is also pretty good for pbp, as there's less crunchy bits involved in character creation (though I was forced to create a system for quantifying "power levels" and a power creation budget); I'll also say that it gives the player quite a bit of freedom in creating their character's powers to have the effect they want. There's a bit of a learning curve on the game mechanic, though, so it's a tradeoff.
That said, if you're into teenaged angst in your rpgs, I'll second the recommendation for Masks. "Damage" is more akin to emotional baggage, but the progression system is good and the rules are geared toward simplicity. Only PCs make rolls; the GM does not.
If the new Marvel Multiverse game wasn't so...Marvelly, I'd say it'd be interesting to delve into, but they're also going with a WotC new-D&D business model, and forcing you to spend $50 for very restricted electronic copies, so I won't recommend that to anyone 'til that changes (even as I'm interested in seeing how the gameplay goes...it seems like a decent setup).
That said, the crunchy bits in M&M-3 are largely in character creation; I can't say the mechanic itself make pbp any more difficult than other 3.5-based OGL systems do, at least in my experience. It may still be the way to go, especially since it allows players a level of freedom during creation to model their superhero idea.