ok first of all excelent job! i love the background it really shows the characters personality!
also cascadance is right, try highlighting the character from the moonlight. it will make the picture glow and have sparkle to it. also white highlighting i wouldnt sugest. i'v tryed it and it looks quite ugly. =/ so if your going to highlight the horse use a lighter shade of the color used to color the horse.

otherwise GREAT JOB!!!!

the background is perfect its not too cartoony and not to real!!! beautiful job!
P.S. i love her dress it's very detailed and i could tell you put a good effort into it!

Wow! The first thing that comest to my mind when I see this is the excellent choices in colors, the whole picture goes together so nicely. Her general porportions and anatomy look spot-on as well.

What would improve the background the most IMO is fewer brushes. It looks like you have definately been using the brush crutch when you painted in this background- I can see it in the grass, the stars, the moon, and the trees. I know brushes are a great shortcut when you're running low on time but a lot of times they don't fit in with the style of the rest of the picture. I.e. your horse is cartoony and the background has quite detailed shapes. Perhaps you could try using the blur tool on the grass and tree. That would make the background tie in together with the less-detailed, cartoony theme. Also, do you notice how the mountains have a yellow highlight where the moonlight hits them? That looks really really good, try putting that highlight in other places too like along the top of the tree and even on the charachter. Again, it will unify the piece and make it all look anchored and together like it is a real landscape that you could walk right into!

Homg. The detail in her clothing is fantabularifficawesomeamazingness! *is jealous of your skillz* I really have almost nothing to say about the charachter herself, she looks awesome. I don't particularly like the lips on the anthro animals, but that's just personal taste. Try a yellow or silverish highlight along the top of her tail, the top and right edge of her forelock, and the top of her chest and along the right side of her outfit. Since the lighting is coming from behind her (the moon) she will have some strong light areas on the sides of her body closest to the moon. A halo effect might also look good here since the lighting is from the back, but then again it might look cheesy. Play around with dramatic lighting and highlights, you have a great base going here! The lens flare fits in well here, I think. Teriffic effort overall!