I had played all my games on slow as well, and very slow before that when using TNK's set. This was the first I played on fast, and I haven't played on normal.
I was curious about it as Tyler said it could possibly be a middle ground between All-Pro and All-Madden. I haven't really noticed to much difference so far, I was just looking for a slight challenge increase without going to AM.
I will say that collisions and bounce offs look slightly better...maybe lol. I will say there is a slight improvement in say tackles/falling down. Where they sometimes look a little awkward on slow they look a bit more normal on fast.
Edit: Playing game 2 in Buffalo now.