Good Effort
Never thought I'd find another good fighting game around here, especially one at this quality. You have a very solid basis here to do something great, and even though there isn't much of a *solid* combo system in here yet, some combos are possible. There's even a reset and some cross-ups, and combos into normals from some Supers w/ the Ninja character.
The framerate is doable but at least for my system (which doesn't have that much RAM mind you) I'm not sure if I'd play this for serious matches online unless I could get it to run better on my end. Button response seems good, though w/o features like links and cancels I can't fully validate that. There are a lot of attacks in here that seem like obvious combos, but the game doesn't count them :S.
Visually it actually looks pretty nice even w/ all the SFIV models being used. I liked the scrolling cloud effect for the background in the hills, if other stages had stuff like that they'd be just as good. For a flash game it looks very sophisticated and the animation is adequate, sometimes *very* smooth for certain Supers (so I assume if u could tighten some coding the whole game could run as smooth as those moments).
I also liked the fact the keys were remappable, and the music didn't bother me @ all. It has a MvC2 feel to it x3.
A few things I'd suggest are: change the input for some the Ultra-like moves (they seem way too easy given they're supposed to be the most powerful moves), remove the double-jump from some characters (personally I'd say remove it from all; I like it in Guilty Gear but not SF-like games), incorporate a deeper combat system w/ cancels, links (both the loose sort that seem to be in here and tighter ones), a further scalable settings feature (I'd love to play this online if I could turn down some of the other features, maybe muting the music for example), tighten up some of the moves (most characters seem to have palette swap moves) and maybe an original gameplay mechanic like the focus attack or grabbing in the middle of combos, roman cancel, something.
Oh yeah, something happened while playing where the game dropped my custom key recognition. This only happened in training mode, and it might be my keyboard but the keys worked fine outside of the game; u might want to look into that.
Overall, this is a very good first attempt but there's room to make this into a quintessential flash fighter, 'cuz there aren't many good ones as-is. This is already there, and it can be even better w/ some retooling.