I am terrible about this, though I try not to be. The biggest turn-off for me is when cover art is bad (in quality) or the font is ugly/cheap-looking or both. If it looks like the publisher couldn't afford good design, I start thinking that the writing inside couldn't be very good.
But as bad as I am, I feel like kids are much worse. For example, I have many a great MG novel in paperback, leftover from my childhood (all those Dell Yearlings!). Because they're paperback editions and OLD, the covers often depict dated-looking children or have some sort of soft-color art not seen on today's vibrant photo-cover shelves. It's very hard to get my daughter interested in what's inside the book, but I bet if I had all new and colorful cover on them, she might be more likely to give them a try. Sigh.
I've always been interested in cover design but fear I'd be bad at it. I wouldn't be able to give a book a non-matching (in tone) cover, as some of you discussed is done, just to sell the book. That sort of thing irks me to no end. Even worse, I once saw an edition of Freaky Friday (the book) with the cover from the Lindsay Lohan movie. Inside, the book was the same great off-beat story as always, about the girl in the NY apartment with the neighbor named Boris/Morris. The cover, of course, depicted the Lindsay plot--which, if you've read the book and seen the movie, you'll know is VERY, VERY different! Yikes. I immediately felt terrible for all the kids who would buy that book and be completely confused by the bait-and-switch.