1) Keep It Simple, Stupid...Sometimes
Sometimes simpler is better and sometimes simpler is just...blah. When I pick up a book off the shelf, I'd like some sense of what it's about. The cover is a great way to give clues as to what the reader will find inside. Fantasy often has a look to it, as do sci-fi and contemporary. Nobody is going to pick up a book with rippling muscles and heaving bosoms and not know what it's about, am I right? But if the cover is too simple, too bland then I'm that much less likely to pick it up. Sometimes it's the more ornate covers that catch my eye, or the ones with more detail and more going on.
2) Fonts FTW!
I wouldn't go so far as to say that I'm a font fanatic, but fonts definitely matter to me. If the chosen font for a book cover looks like someone went a couple rounds with Photoshop and called it a day, it's a little off-putting. (And if you use Comic Sans, I will cut you.) That's not to say that fonts can't be fun. Just look at Jennifer E. Smith's books, The Statistical Probability of Love At First Sight and This Is What Happy Looks Like. Both have totally fun fonts (the latter has the word 'happy' in the shape of a smile), but they don't look sloppy or amateur. The font should suit the work.
3) Your Cover's Blown
Otherwise known as: When stock photography goes way wrong. I'm going to call a book out here only because I really enjoyed it and feel like it can stand on its own, ill-suited cover notwithstanding. The One That I Want by Jennifer Echols was a great book, but it suffers from mismatched cover syndrome. I'd kinda like cute, Japanese-American Max on the front, but instead I get some Cory Monteith doppelgänger who is just SO wrong for the book. There are whole conversations we could have about this—white-washing on YA covers, authorial say in cover content—but I think we can agree it's frustrating when a cover doesn't match what's inside.
4) Content With Content
As someone well past my young adult years, I'm a proud YA reader. That said, there are certain book covers that I'm less likely to prance around in public with. You know the ones I mean: a couple caught two seconds away from being in flagrante. Basically, the YA version of heaving bosoms and greasy pecs. Ugh. If I'm going to read a bodice ripper, you better believe that sucker's going on my Kindle or not leaving the house. I'd rather not have to explain the couple making out on my book.
5) Color Me Bold
I don't know about you, but I love me some color on my covers. There are a few books out this year that have gorgeous, vibrant covers that grabbed my attention the moment I saw them: Taken by Erin Bowman, Transparent by Natalie Whipple, and Splintered by A. G. Howard, among others. But sometimes that whole black and white effect with a single hit of color can be just as appealing. Take Between the Devil and the Deep Blue Sea by April Tucholke, for instance. It's lack of color underscores the creepy and gothic nature of the book, and is therefore perfect.
6) Changing Horses Midstream
It seriously chaps my hind quarters when midway through a series publishers decide to up and change the look of the covers. To the symmetry-obsessed and people who can't handle mismatch this is just crazy-making. Uneven book sizes, hardback versus paperback, completely different theme or scheme... It makes me want to do one of two things: 1) Go out, start over, and buy the whole series in the new covers, or 2) Not buy the rest of the series, but borrow it instead. Now #2 is kind of silly because it punishes the author for something that isn't his/her fault, and also, it causes a whole new problem→unfinished series. O_O At the end of the day, I get why they do this (to provide broader appeal, etc.), but it still irks the heck out of me.
There are other discussions to be had about YA covers—the frou frou dresses, the close face shots, gender* and race issues—but I think I've babbled on long enough.
What hooks you or irks you when it comes to covers?
Coverflip series and the issue of gendered book covers.