Okay, here's my two cents worth.
You can draw better than average, take comfort in that. I'm assuming that you work primarily in digital mediums, judging by the quality of your coloring. First the good:
Your turtles are the best example of your work. I enjoy the crisp, clean lineart. This is clearly you at your best.
Now here's where things get muddy.
Your coloring and shading techniques don't match up with your lines. The shading on your turtles looks "patchy", seeing as they are scattered-about crosshatches. This is an example of contrast in style, and it rarely works. Carry on with what you're good at, which is clean lines, and work that into your coloring style. It looks very much like comic book art, so take a cue from that and use solid black shadows that contour to the form of your object. I recommend you learn how to cel-shade, as it would compliment your already present skill.
Now I'll be honest here; I think your Hulk is hideous. I am aware that it is merely a sketch, but here you completely abandon your clean lines and instead use short, choppy strokes to eventually outline your form. Doesn't look good ever. Now, if you finish your sketch by making new lineart for it, then you have a chance at redemption. Again, you simply aren't good at the soft, painterly style of coloring, so please, attempt a crisper, cel-shading style to appropriately accommodate your strengths.