I haven't played a RPG in-person since d20 SWRPG (pre-Saga) in the early 2000s (and back when I lived in Houston, where it's one hell of a lot easier to find players than where I live now).
The thing to keep in mind about PbPs is that the pacing is much, much slower than non-PbP players are accustomed to. But one of those advantages that swrider alludes to is that you can (and should) take your time in composing replies. You can be very detailed and wordy if you wish (both in "spoken text, that which your character says within quotation marks, and that which you as your character's narrator write as expressing his thoughts and feelings), since you're not under time pressure. Just imagine that you're writing excerpts for a novel (which, in effect, you are). And, to please quasi-OCD grammar-nazis like me, check your spelling and other such writing details.
Another thing that's important in a PbP, even more so than for an in-person game, is for players to take copious notes about their own character, other characters (beyond what's available for online viewing), what info the GM provides both in the IC thread and when answering questions, and anything else you may need to remember. When threads get to be dozens and hundreds of pages long, it's not really practical to wade through them to find something that you forgot. This board does not have a search function for text, like the FFG board does (and Google is unlikely to be helpful, even though it indexes the site at least once a day). So if you neglect to take notes, best case is that you'll have to wait and bother someone else (like me as GM) to remind you of what you should have written down.