A. Unfamiliar with other forum game communities, but I would say freeformschooler, although unlike piecewise he doesn't have his own whole forum subculture (that I know of, unless TWoOtA counts), so maybe it's piecewise after all? ffs has been around way longer, though, methinks.
B. No more than one right now, and there's quite a few hosts, I think.
C. I'm thinking lawastooshort for this one (Roll to Dungeon Quest, Magnificent Timelord, Hitpriest, possibly ROLL TO DO A QUEST), unless... hm, I might have technically played in more GreatWyrmGold games, now that I think about it. One of them (Thawed) was really quite good until it imploded under the bickering, and the rest, I believe, were various minimalists and one Metamorphican Madness. Yoink and his minimalists (and the odd game with a char sheet) might be in a similar boat if I think on it a little more.
D. Might be piecewise, too, in terms of post volume. Urist Arrhenius most recently. But probably Parisbre56 for emotional involvement.
E. I anticipate Special People by Parisbre56 the most turn-by-turn. Can't say I'm particularly anticipating any new games, though I'm intrigued by la's new game. EDIT: Wait, there's Draignean's apparently upcoming game. Kind of excited about that.
F. Urist Arrhenius, maybe? Depends on if a character sheet counts.