You can rate images on both technical and creative merit. You may feel an image has demonstrates great technical ability but the subject matter doesn't particularly appeal to you. You can rate each aspect accordingly from 1 to 5 stars. You don't have to rate both aspects if you'd prefer not to, but each vote will contribute to an overall rating.
Your username will be recorded and will be visible to other users, so please rate honestly and take the time to explain your rating, you might just be rated in return.