My quick thoughts are for you to reinforce the details. Details like the webbing on Spiderman's Suit, or the Hard lines of Doom's armor. Also some more stronger and dynamic posing would have been a good choice.

Now with some more in depth thought.
I think a bit stronger composition with a more dynamic pose would have really helped the impact of the overall feel of the piece. I think something with a little more of the body showing would have been good. Perhaps doom holding Spiderman off the ground by his neck and seeing Spiderman struggling to get free.

The anatomy could use some work. Its not too far off, but it could use some more consistency in places such as Doom's hands and arms. Also Doom's neck and head seem thin and small compared to the rest of the body. I also notice that Peter's right shoulder is very small and puny. Spiderman may be a little on the smaller and scrawny side, but he does have some good solid muscle in the right areas.

The coloring could have been much more solid. Although it is breaking about par, I think you could have pushed your color pencils to have a bit more rich color. On that same hit I would like to mention some lighting and other effects. I don't see and definite or solid light source. I also see little to no shadows or shading. You really need to get a strong Light to case some nice hard dynamic shadows on the characters. IN that same sense this would give you the most awesome opportunity to play with Doom's armor had its reflectivity,

As I mentioned in the quick thoughts I think you should really get the thicker lines on the costumes. This also applies to the line work.
A couple minor changes and this could become a pretty bad ass piece. At the moment you have a start and I will pleased to see you come out with more comic book related pieces.

spiiderman is really hard to draw. I like your drawing of spiderman enough but because of his costume it is really easy to see where you went wrong with the shape of his chest and head.
the spiderwebs have to mimic the shape of his body.

also his neck is off. you might say that it looks that way because dr. doom is squeezing him, but to get his neck that distorted doom would be practically killing him. crippled spidey after this.

doom is pretty solid. I would make his left hand stand out more. Because it blends in too much with the arm behind it.

all in all good work! spiderman is insanely hard to draw and I like the folds, and value change on doom