Her back looks long to me. Either she's built slightly downhill or she's got a slight roach to her back...or something. Her coupling through her hip looks odd to me.
Her neck is set high on her chest, which makes her look ever so slightly ewe-necked.
She does have a beautiful angle to her shoulder.
She is ever so slightly back at the knee, as Tryst mentioned.
I think she's got a cute head, but this is coming from someone with a horse that has a mutantly large head. Lol
Something about her back legs bothers me, but I can't put my finger on it, especially with her tail in the way. Her right rear hoof looks odd, as well (I know it's chipped, but it looks bigger/more flared than it should be).
Overall, she looks like a sound, sane, usable little mare. I would make sure to do plenty of hill work to strengthen her core to support that long back.