As it was a personal comment on FB, I won’t quote her by name, but here it is:
“Some books you can’t judge. Most you can. The fact that it is most is what makes the aphorism annoying to me. It might as well be an expression like “you can’t eat dinner at a table”. Yes, most certainly there are times when you can’t eat at a table (libraries, museums, tables covered with things, tables in other people’s houses which you aren’t even supposed to be in, you have no food, etc.), but as a general rule, tables are places you can eat. Even using your example, protagonist on the cover can often, though not always, tell you quite a bit … Does the protagonist appear to be male or female or gender neutral? If male, is his shirt on or off or on but deeply unbuttoned and/or ripped? If gender neutral, do they look like a monk? If female, what is she wearing? Is it practical or modest or sexy or skintight or porn-armor or actual armor? If sexy, is it in a mode traditionally targeting a heterosexual male audience or a female audience? Is she wearing Louboutins? Is the protagonist armed or unarmed? Do they look frightened or bold or happy or shocked? How old is the protagonist? What is in the background? Lightning? A house? Does the house look creepy or inviting? A city? What’s the color theme? And those questions are just assuming the person is alone on the cover.”
Case in point, this Paul Lahr cover and text tells us it’s SF, and it’s got some sexual content. That’s not a bad start to hooking a reader.
And much as I dislike People Staring At Me book covers (this one by Blake Morrow) this one clearly does tell us something: the protagonist is female, she’s ready for action, and she’s in some kind of wetlands area. And she’s goodlooking.
And this cover by Leo and Diane Dillon says “something weird and supernatural” (RIP Ursula LeGuin).
Rights to all covers remain with current holders.