Feels off-balance to me... A bit top-heavy. I think a bit of fill flash from the phtog's right side would help to bring out more detail in the subject's coat.
Beautifull horse, Maybe could have done with a little tighter crop so we could see the horses face better and maybe some more depth of field. There's something in the background (grey pole, like a pole for power lines) could have cloned it out, but its not realy distracting because of the shallow dof.
Great shot. I love the tone and feel here. Personally, I feel there's too much negative space, and capping that off, there seems to be some fench colored poles back there that are distracting. You might consider later cropping just below the bottom of the one on the right.