There are a lot of things I could nitpick on here, but when I step back and look at the big picture you mostly have the proportions right and things look nice and steady. It seems like with a bit of tweaking you could fix this up a lot.
One of the odd things I noticed about the characters' postures is the way the knees don't seem to bend the right way. Either they just don't bend at all, or perhaps the slender proportions of thighs and calves make the positions seem a little unrealistic. In addition to the zombie's left leg being a great example of this, the girl's right seems not to have quite ENOUGH of a bend in it, and other minor things like the zombie's right calf just seemed slightly -off- to me.
The slight blur on the edges and on the floor reflection are nice and bring focus to the characters and the center of the image, although I felt the reflection was just a little too clear for my liking. Still, the bright colors maintain the focus; in particular, the girl's pink hair and pants give her a certain vibrant appearance that stands in contrast with the more grayed-out zombie.
As far as other colors go, the girl's scarf seemed to blend just a little too well with the tank top, which, for a moment, gave me the illusion that they were the same piece of clothing until I zoomed in. Perhaps a slight color change there would have been welcome.
Finally, and most importantly, the first thing I noticed about this picture that kind of annoyed me was the depth of the hole in the zombie's torso, or rather, the lack of depth. The clothing isn't bloody enough, but more importantly the hole makes him appear paper-thin. Simply adding a nice big crescent there would have made it look much more realistic, even if it would have made the hole look a bit smaller.