Great horse shots are a favorite of mine and this one is a great one. Your central composition balances well with the off-center head, mane, and body. The DOF (as others have said) is just right as is the exposure. The eyelashes are one detail that makes my point.
What may have hurt you in the contest itself is the title -- I think the shot deserves a higher score. I didn't vote in this challenge but I would have scored it among my favorites with a 8 -9.
Making this better? That's a tough one -- one suggestion would be a slightly tigher crop on the left side eliminating the lower left "streek" but then you would have lost perhaps too much of the tail.
Welcome to DPC - I see you like wildlife and uniquely cropped images -- I enjoyed looking through your portfolio.