Sorry I didn't ask permission, but I did a quick paintover to iron out the kinks.
The first change I made was to the arm. I re-proportioned the bulks of muscle. Sharpened up the highlights on the triceps and downplayed some of the other masses of muscle. The forearm originally had a lot of mass in the backside, but when you extend your forearm completely it practically goes flat, so I drowned out the extra mass with highlight to adjust that. You originally had the deltoids leaning back, almost, whereas the muscles actually angle forward. I bulked up the bicep and increased the depth of the hollow inside the elbow.
The next big fix was the torso. It had a lot of mid tones in it, making it look kind of flat. I bulked in the anteriors (the little muscles that line the ribcage) since they were just kinda lines before. I significantly darkened the side of his body to give it more depth and "pushed" the abs more to the front by adding darks beside and underneath them. I broadened the highlight on the back to give it a broader, flatter appearance. I put more emphasis on his "love handles" and pushed the profile of the abs forward, which follows down into the crotch, which should technically have a slight bulge. The pecs were pushed forward, a nipple was added (lol), and I emphasized on the striations in the pecs that become more apparent when you pull your arms back like that. Also increased the volume of dark in the underarms and beneath the clavicle to separate it from the chest.
The head was an obvious fix: originally you had the profile leaning way backwards, so I fronted it a bit near the top. I brought the forehead forward, pulled the chin and lips back, gave him a little more of an adam's apple, bulked up the underside of the chin (which kinda inflates when you lift your chin) and defined the shoulder muscles. As a small detail, I added folds to the back of his neck.
Originally, Vegeta's butt was kind of flat and had small slivers of highlight. I erased the small sliver along the bottom, gave the butt some more form and added that blot of light toward the inside of the buttock. When you have really defined buttocks , they create a dimple like that. I also darkened the crotch line.
The Hulk was almost perfect as is. The only thing I did was bring the center point of the clavicles forward, since his chest seemed to be facing the audience more than his head originally. I bulked up his pecs and made his body curve downward more to give the illusion that he's hunching forward instead of floating (like Tomsan said).
On a compositional level I noticed a lot of negative space in the bottom right, so I added Hulk's fist. This gives you an opportunity to add depth by applying atmospheric perspective to the fist, improves the sense of scale, adds character interaction, emphasizes Hulk's disposition and gives him more presence. It also grounds him, giving him the illusion of standing on solid ground instead of floating.
Anyway, that was a long rambling post. I hope it helped, though.