Yeah, when I first read the 4e books, I thought about how it would work in a PbP. And the short of it is, 4e does not work well in pbp.
Skill Challenges will slow your game down, but not as much as some other 4e factors. Simple interrupt and reaction powers will slow your game down at least as much. A fighter who marks opponents will have to be asked each time if he wants to make an AoO. And so on, and so forth.