Judging a book's cover

I'd planned a series of posts on the benefits and pitfalls of entering and judging RWA contests, but a discussion over on the Word Wenches blog has put covers on my brain--specifically the cultural ramifications of the different covers developed for the same books in their UK and US editions. So over the next couple of days, I'll be comparing the US and UK covers for Naomi Novik's debut fantasy novel His Majesty's Dragon/Temeraire and for Bernard Cornwell's most recent Napoleonic-era military adventure novel, Sharpe's Fury. In both cases I prefer the American cover (though the British covers are appealing, too--this isn't about beauty vs. ugliness so much as subtle differences in the standards of beauty). And I mean to try to figure out why.

But for starters I'm going to lay out my theory of the purpose of a cover. None of this is original to me. Authors talk about covers all the time, possibly because they/we have next to no control over them, and I'm pretty sure I'm paraphrasing Jennifer Crusie and several others here. That said, this is how I analyze a cover's effectiveness:

1. A cover should catch the eye of a browser in a bookstore and make him/her pick up the book. Therefore you want some kind of visual hook, something to make the reader think, "ooh, pretty," or "ooh, that looks like my kind of thing." Granted, if I love an author, I'll buy his books no matter how dull or garish the covers are. But if you want to draw new readers and maximize sales, you need a look that will somehow pop off the shelf or display table and make them stop and pick up the book.
2. A cover should sufficiently reflect the contents of a book that the reader doesn't get cognitive dissonance when she starts reading. Cartoony, bright covers don't belong on dark, gritty stories. Contemporary images are out of place on historical fiction. Sexy books shouldn't have overly sweet covers, or vice versa. Etc.
3. A cover shouldn't be so garish or tacky that I'm ashamed to be seen reading it in public. This mostly applies to a certain sort of romance novel covers, but I've seen books from other genres, notably horror and fantasy, that I wouldn't be comfortable whipping out on a plane or a Metro bus. If you stop to think about it, this is a marketing issue. I've often had coworkers or fellow commuters ask me about my book--do I like it, would I recommend it?--but if the cover is so embarrassing that I only read it in my own house, trying my best to hide it even from my husband, that publisher/author has lost my word-of-mouth advertising.