We've all seen the alluring YA covers of the past year or so - they seem to focus on a woman or person, usually facing the other way. They're typically wearing an outfit blending into the scenery and coloring of the books. The focus isn't really the scenery, although sometimes that can be utilized to focus on the person.
Here are some shots of popular books where the face is covered but you can find similarities:
If you notice Anna Dressed in Blood, the girls hair is black and white, along with the title and the background and dress. The only color are the red flower petals and some red low-lighting for the font. Likewise, The Mephisto Covenant and Fallen in Love also blend the colors of the background into the dress and coloring of the person. The bigger exceptions are Immortal and Hourglass, where Juliet Immortal actually has a scenery that DOESN'T blend in with the girl, and Hourglass has a neutral dress but wanting to pull attention to the different gradient shades of blue in the cover.
Personally I'm really digging this trend in YA of 2010 - current. Instead of the big poster looks some of the books were having (that looked like movie covers with the face of the person and a movie-type background), these are all artistic, eye-catching, and have a mysterious quality about them since the faces are obscured.
Since many of these books have been voted on Goodreads as favorite covers of the year, it looks like many readers feel the same way.