I have no idea.
Okay, I have some of an idea. Like a third of one.
I imagine there are stats and research as to what kinds of covers attract more readers. I haven’t really thought about it until recently, although I do know a few kinds of covers I particularly like. Most of them tend to be colourful, either with an interesting design or characters on them. Some of my favourite covers are on the Discworld paperbacks. I adore the cartoon weirdness that’s going on (although I get that that style doesn’t suit all books). And, honestly, I’m also pretty partial to anything with a dragon on it.
And now that everything is bought online (including the souls of the damned, neatly bottled in homemade jars you can buy from etsy) the thumbnail is also an important factor. It has to be something that’ll catch folk’s eyes as they’re scrolling through the online storefront. Right now I’m working mostly in a digital space, so I don’t have to think about “how would this look on store shelves.” That means the image has to be eye catching in a small thumbnail format. It’s something I keep at the back of my mind while thinking up cover ideas, but I don’t stress too much about it. I’m not a designer, and the fact is that if I became overly focused on this I’d just drive myself crazy.
What should the cover convey? An emotion? Characters? A potential plot point? All of the above? None of the above? I would say what the author thinks is most important, and in self-publishing that’s probably the case, but not so in traditional publishing. It’s my understanding that traditionally published authors have little to no say in their covers, so who knows what kind of dark magicks the publisher’s marketing team invokes to conjure up a cover.
You could have some characters on the cover, and something in the background or foreground that’s relevant to the story, or reflects a particular event. A sci-fi novel with a futuristic cityscape is interesting, and could be awe inspiring. But that same cityscape mid explosion could make a potential reader go “the hell is this about?” (There should probably be an actual explosion in the book, otherwise it’ll feel like a letdown.)
Frank Frazetta, who is one of my favourite artists, used to do some absolutely amazing covers for old pulp novels. Frazetta’s covers must have sold many a book with the shear promise of spectacle. But a lot of his covers depict events that don’t happen in their respective stories, and as gorgeous as those covers are, some readers must have felt a little letdown. Which is why I don’t think it’d be a good idea to throw something on the cover for spectacles sake that doesn’t reflect an actual event/scenario in the story. Even if the story is great, and the reader loves it, there will be that little part of them that goes “but the city never blew up.”
It’s what’s inside the book that matters most, but the cover can be the first impression, that initial impetus to see what’s inside the book. It’s important, but not that important. I've barely started to figure out the formula, and I might even be following the wrong string of equations. The only way to find out is to keep experimenting.