I have to say that one series I just couldn't get into was the Twilight series. I tried, but there was just something about it that prevented me from enjoying it. So, I really only got about halfway through the first book before I gave up.
My favorite book covers are the ones that are simple and not too busy. If there is just too much going on on the cover, it bothers me and I will usually skip over the book for something else, unless it's by a favorite author. Another type of cover I only pick up if it's a series or author I like is a cover that is too cartoony.