In terms of art, here's a breakdown:
-Art style/detail varies wildly from fighter to fighter, making it visually non-consistent.
-The animation in general is bad. We all know that they didn't want to add any extra frames of animation and we understand they want to die on that hill, but you can tell they didn't hire animators to do this. Look at some of the idle animations. Look at Ryu's feet. He's supposed to be bouncing up and down much like you see any fighter, but his feet are all over the place. Furthermore, the human eye can tell if something is the same drawing stretched/skewed/rotated. There's a ton of that here and it makes it look cheap and lifeless.
-The art in general for the characters is bad. It seems they couldn't decide whether to 'interpret' how the pixels work together to form shapes, or to just trace lines verbatim. Some characters had their poses/proportions altered (Ryu definitely) while others had adherence to the original sprite good art be damned. This is especially evident on Cammy, where she looks just awful. I know they didn't want to risk going against original sprite sizes/hit-boxes, but they seriously just could have lowered the bottom of her crotch, and dropped her knees to compensate, and she would have looked way more human. Even then, some of the tracing was done poorly. It's hard to tell sometimes what forms are supposed to be in front of each other. Really flattens the characters.
-By far the worst part that brings the whole thing crashing down are the backgrounds. Not only do they look arguably way worse than the originals, they interfere with game play. The fighters are all far less detailed, with basic shading and colored lines. The background characters all have detailed faces, more detailed shading and thick black outlines. Go to the Mexico stage and move the fighters towards the crowd in the center. Who's in front? The fighters, or the crowd? This is not only unpleasing to the eye because it makes everything so flat, it also makes it hard to tell where your character is during a fight. It's actually possible to lose track of where your fighter is or what your opponent is doing. The old art had a sense of depth where it looks like the fighters are in a world that you could walk around in. On top of that, you could always tell where your characters were at. That's because the old art has atmospheric perspective and depth of field in terms of how it prioritizes detail and color. The fighters are the most detailed and pop out, while the backgrounds get less detailed and more muted the further you go back.
It's a product of low budget and fast deadlines.
