Well, blood was never a major part of fighters...although on the surface they look the same, there are differences in them all.
Speed is one, for sure. A game like SF3: 3rd Strike can tend to be pretty damn fast paced (japanese tournament vids ) whereas a game like capcom vs snk 2 goes generally seems to move slower.
Combo engines are a major one. Every game has a different engine, however slight the changes may be. They make a huge difference over who's good and bad in a game.
And of course, characters themselves. Even Ryu and Ken have some major differences from SF2. Even if the moves look the same they may work differently. There's a reason why Ryu is better then Ken in SF Alpha 3, and why Akuma is better then both of them in the same game.
Finally, the difference is again, the head to head aspect. People like being able to defeat their opponents in a match more then they do a scoreboard.