I've read part of it before I lost track of where I was reading when I missed a few chapters while computerless for awhile.
1) her arms seems disproportionately long for her body and it bothers me.
2) if you didn't know the story at first glance it looks like the brightly colored swirl is coming from her hair instead of her mouth.
3) if you do know the story you wonder why her mouth is not open
4) I liked the lettering of the title except for the fact that the W is duplicated. Some people might think of the title as "W. warbreaker"
5) I wonder if guys would be turned off a bit to picking it up because the cover might look like "a girl's fantasy story". I've know guys who do that. They also refuse to read anything with dragons because they think those are fantasy stories "just for girls".
6) I miss the depth of field in the art that's been on your other covers. I kind of wanted something in the background to add to it? set context? I don't know.
7) seeing this image after reading a part of it I can see how it fits in with the story (and also helps to bring back the story since it's been awhile)
I like to see a little bit more blood on the blade
$.02 given (for what it's worth)