I've been somewhat of a fan of fighters all my life, but SC4 has been the first one that I've "taken seriously." The Virtua Fighters looked boring to me. Like Jaxel said, it's got no flash. I mean, even high-level play looks boring. And as for Tekken, well, I played it for fun when I was smaller, but it doesn't grab my attention as much as others. SF4 is almost too difficult because of the just-frame-esque combos.
I think I really like SC4 because of the complexities. Even on defense, you gotta decide to block high or low, GI, try to break a grab, or punish. I don't mind the CF system, as it adds another small level to the game and forces turtlers to actually try to GI or something. Each character, for the most part, is unique, both visually and move-wise.
Have any of you played Naruto: Gekitou Ninja Teisen 4 (GNT4)? It only came out in Japan, and for a Naruto game, it was really fun. It had certain just-frame combos, but it also had something called "Y cancels," which allowed mid-string cancels when your opponent is in mid-air. It had the flashy specials, which were simple to do. It had newb-friendly combos. However, behind all of that, the high level play wasn't bad either. Combos could get really complex and end in specials. Out of the 25+ cast, only two really stood out as top tier, and the rest were pretty balanced. It's easily one of the best fighters I've played.
I hope the next fighters aren't just geared towards the casual gamers though. On the other hand, don't make weak three-hit combos impossible either...