The range of graphic styles on teenage book covers is huge, as I'm sure you're aware. I'm in my forties, but I (try to) write for this age group. I think what they look for is maybe something that reflects the theme and tone of the book, be that light and chatty or more serious and deep.
More than anything, teenagers need to be able to identify with the cover design. Check out Celia Reece's 'Pirates' and 'Witch Child'. Both covers depict very fetching young girls and are almost erotic in nature. I wonder if they would have sold so many copies had the pictures been of old hags and brawny women pirates. Would a girl read a Jacky Wilson, for example, if the book had a cover design, say, like that of a Catherine Cookson or a Penguin Classic? She mightn't even lift it off the shelf to look. On the other hand, a girl who wants to read something similar to Austen or Bronte isn't likely to be drawn to the cartoon designs that seem so popular just now (in fact, if you take a look at the covers on the latest Jane Austen editions, you'll see they've been given a very modern design. It wasn't popular with some of the industry, but the publishers have done it in an attempt to appeal to readers of more modern fiction).
You could do worse than ask your local secondary school or library if you could speak to some of the teens there. It is very relevant to the national curriculum in terms of both English and Art & Design. You'd probably be a welcome visitor.