Love what you are doing with the face.
The musculature blending on the lower front seems to be odd, which I know is silly considering what it is, because you are trying to create a new transition between the abdominal group of a human and the pectoral and deltoid region of a horse. If you look through images of centaur, this is often the most complicated part, and many artists simply cover that up completely in order to not deal with it.
Here are two examples that I found that made it work, so that it looks more like a creature would actually evolve.
It seems that in both images the artists have chosen to blend the lowest segment of the human abdominal group with the pectoralis of the horse into one smooth muscle.
Good luck, that is the only odd spot that I see, its looking great.
