*Dredging up incredibly old topics*
I very rarely buy books (in fact, I really only buy books that I've already read, and know I like X3), but they can affect if I buy or not.
Recently, I'd been sort of feeling like I might reread the Harry Potter series again, and was considering buying the ebooks for kindle (of the entire series, I only had a few of them, and one was a huge hardcover, which I knew I wouldn't be able to get myself to tote around X3), but when I saw the new covers by Kazu Kibuishi, I decided to get those instead. (The art is beautiful. You can see the new covers here: https://chasingtheturtle.files.wordpres ... buishi.jpg
As for reading, I think the covers affect my choices less than they used to, but they will very often affect which books I pick up the shelf when I'm browsing.
However, I've found my tastes in covers (which covers draw me in, and which make me keep on moving) have changed, so in that sense, it's hard for me to define 'good' covers and 'bad' covers, at least when it comes to bestsellers and other books that have gone through a long vetting process. (And I've found that some of the covers that draw other people in are the types of covers that I would automatically skip. For instance, for some reason I tend to pass over covers with bright colors that appear to belong to the humor genre, and also old-fashioned covers. Plain covers that don't give much of an indication of what they're about I often miss, too.) But at the same time, sometimes plain covers, particularly for non-fiction books such as those written by Malcom Gladwell can be attractive to me, because it's all about the title and subtitle, and whether those seem to deal with an interesting concept. But I'm sure that's also because of association, because I've come to like those sorts of books now, whereas I didn't when I was younger.
I guess for me a 'bad' cover constitutes a cover that makes a book appear to be in a genre it's not. In that case, the book attracts the wrong audience, and puts off the audience that the book would really do best with, and so the people who end up reading the book are bound to be disappointed, and the people who might really like it miss out.
But I guess because I like to draw, my favorite covers are ones that feature beautiful artwork, realistic enough to bring the characters/setting from the books to life, with good character designs that really look how you might imagine the character might be. As I said, I really like the new Harry Potter covers, but I think my favorite covers growing up were those of the Animorph series, particularly those that feature the aliens such as the Andalites or the Hork Bajir.
Here are a couple of my favorites:
The Andalite Chronicles:
The Hork Bajir Chronicles (this one I think was on the back cover, or maybe the inside flap):
I guess this is exactly the kind of artwork I've always wanted to do, art (such as graphic novels) that tells a story, or portrays a scene from a larger story that the audience knows. So those are the kind of covers that I really pay attention to.