Thor will probably primarily contribute special effects and outsider humor (and eye candy for those who appreciate huge arms), Nick Fury will basically be the Maria Hill of this, and I think Wanda may actually get a bit more emphasize than her brother. I have nothing to back this up, it's just a feeling. I feel as though QS will get the Hawkeye role of this one.
Steve is fairly popular and they're probably going to want to do better with him because of WS, so I think he's going to be a big player.
Tony for obvious reasons will, for better or worse, be almost the prime focus.
Bruce has nothing else going on, so if they want to further pitch to the studio that Hulk's popular enough for another solo film, this is their only chance until maybe Avengers 3.
The mistakes of the past will be forgiven with what Hawkeye has. And Widow by extension will likely get a prominent part.
and finally, I fear that Vision may be more or less be a dues ex machina, so he may have the combined importance and screen presence of Selvig and The Other.