I think that your color placement is really great. Having the transition of colors form Black Widow's suit to the Hulk's pants to the red in Captain America's shield to Thor and lastly to Iorn Man creates a great path for the eye to follow. However, I feel that Spiderman is poorly misplaced and actually diminishes the composition of the picture. I could have been completely removed from the picture (unless you placed him there as a sentiment for not being included in the avengers movie, and if that's the case then well played). I do enjoy the expressions on each of the characters faces.
All I have to say is that you are a great artist my friend. You used a tremendous amount of detail and the color is truly magnificent. The best thing about this picture is that it doesn't make you look anywhere else, all your focus goes to Thor's lightning hand first and then to the Hulk, to Hawkeye, to Black Widow, Captain America to Iron Man to the rest of the picture. Well done, well done. Also, I noticed that you included Spidey in the background. That's bittersweet isn't it? Spiderman would have been great to see in the Avengers, but it worked just as great without him. what's your opinion on that?
I love how the Hulk looks a lot like Mark Ruffalo, especially with the hair. Details such the impact on the ground where Hulk's feet are to emphasise his size look great. I can just imagine Spidey zooming past the team and heading first towards the aliens in this scene, swinging at great speeds; it's just too bad that didn't happen in the film.