Yeah, lighting can be a real toughie. I tend to oversaturate outdoor scenes and end up with ugly shadows, and I make my indoor scenes too even, so that the light always looks artificial.
I know that even on a cloudy day, the lighting from above tends to be brighter than the ambient-lit shadows, which the eye adjusts for but the camera doesn’t. If I had to pick one thing that makes the lighting look not-quite-on in this image it would be the hard shadow on the crab. The diffuse lighting in the rest of the scene has the cloudy look, but it almost looks like there’s an aritificial light close to the crab. An area light with a spread out shadow (lots of samples, far away, use ray traced shadows, and be patient :)) might give you more of the look you’re going for in that type of image.
Somebody else in another thread recommended this web site. I bought the guy’s book, and I’ve really learned a lot about lighting in general from it, and I really recommend it.
Still, I think it’s a great picture, and I’m still chuckling over the look an the crab’s face.