I would like to begin by saying that this piece looks very nice, and I'm not an expert, so I wouldn't be offended if you didn't listen to a word I say.
The first thing that catches my eye is the cloth material. Particularly the shirt and jacket. They're both sort of glossy and shiny, perhaps if you lowered the spec on them and added a woven sort of texture to them, they might appear more realistic, which I think is what you're going for. Second, the flesh tones of the rabbit stick out to me too. There are pinks to the tones, but I think those mainly come from the rim light you have placed behind him. Skin, especially zombie skin, has many varying hues to it. Blues, greens, yellows, etc. I'm pretty sure you know this already, but you chose to keep it white 'cuz he's the white rabbit from Alice and Wonderland, but, again, I feel some variation could "liven" this zombie rabbit. Third, I think you have some slight depth of field goin' on which is nice, but I might push for some motion blur along with it (very slight motion blur) in the direction that he's moving in. 4th, the pose is awesome, but I feel like it could be better. More twist to the torso, or having the shoulders slant in the opposite direction as the hips usually helps add dynamism to a pose. Right now, he's running with his chest and pelvis pretty much facing forward (there is slight twist) and the shoulders are pretty straight across. Lastly, if you really wanted to push this thing, you could add some atmospherics. Maybe a very light volume light, or random pieces of cloth stripped from his decaying clothes fluttering about, or perhaps just plain ol' shiny dust could really help him feel like he exists in a world larger than what is simply presented in the picture. It really helps the depth of field effect as well to have particles floating about.
Well, anyway, that's what I've got. Take it or leave it.