Ross pretty much covers it. The airbrush tool isn't the problem; it's how you used it. Everything's just sort of "mushy" -- no real shape definition. Take it further, commit more, get in there and do the detail work that'll really bring out the musculature and create the form.
When dealing with a big form like this, the first step is to determine the light source. Where's the light coming from? Where are the shadows? Judging by the crosshatching under the muscles, it looks like the light is coming from above and slightly to the right. That means that the light will hit on the top right area of the body -- and of each muscle -- and fade to shadow at the bottom, and slightly to the left.
The image below gives some direction. The yellow arrows indicate the direction of light hitting the form; the blue arrows indicate the shadows. The green swoopy lines indicate the
strength of the light -- the light will hit the top of the arm or chest first, then fade down the length of the body. I just indicated the major body shapes (head, neck, arms, chest) with the green arrows because those are the most obvious major shapes.

When breaking down a large form like this, work from large to small. Tackle the big shapes first -- the whole arm, the whole torso -- then work your way down to each muscle, and the small details. In this next image, I brightened up the red first, then began airbrushing in the shadows with a very large brush and low to medium opacity. This blends things nicely. Notice how I'm not getting too nitpicky with details yet; all I'm doing is establishing the overall, general light source.

Next, I'll choose a lighter red, and start working a little smaller. (Odd -- I must've set this file to a different color preview. Well, you get the idea.) Instead of doing the whole arm, I'll work on just the shoulder, then just the bicep, then just the forearm. I'm not yet picking out individual shoulder muscles, though -- not yet. I may go back and re-shape some of the shadows at this point as well.

Now, and ONLY now, are we ready to do the detail work. I'm still keeping the direction and strength of the light in mind --
always remember where your light source is, and keep it consistent! -- I'll start working on each individual muscle and vein. I've chosen a slightly harder-edged airbrush to bring in some details inside the muscle forms (I used a brush set to 20% hardness). Even though I'm doing detail work now, I'm still making sure that the highlights are brightest at the top right of the muscle, and the shadows are darkest at the bottom left of the muscle. I'll also bring in some
cast shadows, such as under the nose and brow, and where the chest casts a shadow onto the left arm. These help to really define the shape and indicate the strength of the light. I put a little palette in the bottom left corner to show you the four reds I used on Hulk's figure.

This line art is so open and forgiving, that you could do a million different things with Hulk here. How would you approach this if Hulk were in a thunderstorm? Or a sunset? How do you alter your palette, and how do you treat the lighting differently? Think about these things too.
Hope this helps!
-- Laura