Most of the "grown adults" I know play Smash Bros and only Smash Bros... I tried introducing them to the SF series but they weren't interested at all, and I guarantee VF is much too complex for them. That was back in college, and I only know one kid over here still into fighters, but a couple of my other friends expressed interest in learning Smash Bros when Brawl comes out. I don't care much for playing online, it feels so antisocial to me (especially on weekend evenings, the only time I really have a chance to play online coop), and its definitely more fun to play with friends over (either coop or competitive)... playing online with some random redneck in Oklahoma doesn't do it for me.
I didn't like Prime either... awkward control setup and wayyyyyyyyyy too much backtracking. I don't like the GTA series much at all as we all know by now, the other games definitely seem to be geared more towards the single player. Halo 3 should be pretty fun though. Owning pretty much every system aside from PS3 (still absolutely worthless in my eyes) means I don't have to worry about being bored at all this year, but I have a feeling I am gonna be playing a LOT of Brawl, since I've had such good times in college with the first 2 Smash games (i.e. 40 kids, both guys and girls, at my friend's house eating shitty Buffalo pizza and good Buffalo chicken wings and drinking and stuff), and I've always liked the series mechanics; easy to pick up and play, with quite a bit of depth, that doesn't require memorization of arcane combo strings or anything and lots of little nods to retro gaming.