Some advice for getting the setting/feel across is to make sure you get some players who are into/comfortable with the setting/feel you are going to play with. Since this is a written medium, in some ways it can come off like reading a book or novel, that is that only certain people are going to pick up certain books.
Also, you usually don't have to detail your story exceptionally far in advance (although vague ideas can be good). Since PbP is slower, it's easier to deal with situations as they come up if your players spring something unexpected on you (often the case). In your posts your descriptions are what you use to convey the feeling and setting to your players, using language that fits helps get that across. If you don't have a large repertoire of vocabulary that'll fit your setting, use dictionaries and thesauri to help fill it out. In the same relation that it is similar to writing, using the same word that you just used a line ago can get repetitive (unless you want that feel).
Check out some of the games in the Playing the Game, I generally recommend those run by Isida for a standard style. Also, check out nonamazing's Beneath the Ice for a slightly different approach in making it a bit more like a story being told.
Conversations will often be slow, although sometimes you can have characters quote conversational text that they are responding to. In that manner you or your players can sort of interject without having it all come out like long drawn out monologues.
Okay, enough rambling. Good luck.