Initially i would like to state that i have not read Warbreaker yet. I have downloaded it, and am currently debating with myself weather to read it now, or wait till it gets published...
Then being said - on the the cover-art
Realising it is only a sketch i have a few comments (and some are repeats of what other have already written)...
1) If you cut the picture in half horizontally, it could be seen as if she way lying down. That coupled with the beginning colour of that thing out of her mouth is red if think it looks like blood and she is defeated in some way - she has also lost grip of her weapon
2) The 'k' in warbreaker really annoys me. I'm not sure if it's a strange 'k', 'r' or 'h'. I would rather that the title was written using the font of the 2. 'w'
3) Does the 'w' impart some significance, since it is represented 3 times on the cover alone. The 2 are in connection with the title, and there is a 'w' hidden in the thing coming out of her mouth
4) what is the thing coming out of her mouth ? - if i were to analyze it, i would say she's dead (the red beginning) and then the colour changes to that of the rainbow (happiness, afterlife ?) which is kinda happy... So perhaps her death accomplished something ?
5) the string of rope tied to the hilt of the sword gives me an Asian/samurai impression...
6) As others have stated, it does bear a tone of femininity. Of being targeted more towards the female half of the population.
That was my off-the-top-of-my-head impressions
Would it make me buy it ? - No
Would it make me read the synopsis - Maybe. The femininity of the cover-art would mean that i should have the time and be in a 'browsing-mood' To flip to the synopsis and read it. If i was just quickly browsing to see if something would pop out - this wouldn't