This is my take on PbP:

-I like classic D&D modules, In my opinion, they tend to work better than sandbox in P&P because sandbox becomes very disperse very soon. Then when the groups has built stability and time, we cold go sandbox.

-Themes are ok, I don't mind playing and all kobold or all gnomes, or a pirates game. Nor do I mind playing a diverse game.

-I don't enjoy half as much the character creation as I do the actual gaming, so you won't be hearing any complains on my part on the character creation aspect. If you say core only 3.5 with 22 point buy and no wealth, I'll be as happy and maybe happier than ALL SPLATBOOKS, HOMEBREW high optimization 40 point buy. Then spending 1 month making a character than never really gets to play.

-That said, I prefer something that seems a bit focused from the beginning. Not necessarily thematic, but focused.

-I also think that some after-creation-selection flexibility works best, adjusting to the actual adventure. If you realize your samurai sucks (you should know it) after the first encounter, may be the DM cuts you some slack an lets you adapt a bit.