There is one, and only one, thing that a good cover does. It makes the right readers stop, and look more closely at your book. Standing out a little is good, but standing out too much is not. Good design is necessary, but not sufficient. A few common sense approaches actually hurt sales, and should be avoided.
If the wrong readers stop, this does you no good. It may even lead them to buy your book, under the mistaken impression that it’s the kind that they like, and then write a really nasty review. That’s worse than making no sale at all. If the right readers don’t stop, they’ll never see the descriptive copy you’ve labored so long over, and never buy the book.
I’ve heard a number of new publishers proudly proclaim that their covers will make readers notice, because they’re so different. This is a classic newbie error. If your cover stands out too much, it’s probably too different from your book’s competitors to signal that it is in competition with them. The cover must use the same sort of visual signals as other successful competitors to your book use, even as it uses them slightly differently. If it looks too different, the message that “this is a book for you” will not be communicated as your readers’ eyes sweep past it.
Experienced designers of magazines, or product packaging, or whatever, often think that they can do a better job than the normal designs used by book covers. They want to apply the things that they’ve learned in these other areas. Sometimes they’re right.
It’s still critical that the cover “look like” a [fill in the blank] in 1 to 3 seconds. If you book is alternate history with a military bent but instead it looks like urban fantasy, then the quality of the design and the ability to get people to stop and look are irrelevant. You’re either not going to make the sale, or you’re going to make a lot of sales to the wrong readers. Not a good thing either way.
It’s common sense that your book cover should accurately and prominently portray the main characters and the stuff around them. Unfortunately, common sense is wrong for most types of novels.
The sad truth is that your reader isn’t going to care about your character, until your words make them do so. You care, they don’t. Yet. They take it for granted that IF the book is of the right type, and IF you’re a good writer, THEN they’ll care. What they’re trying to learn from the cover is “Is this the right type of book? Plot? Theme? Style? Genre?” Those are the things you can answer, by using the right iconography, in a matter of seconds. Sometimes, that means that the lead character will be on the front (romances, some types of science fiction or fantasy). Sometimes not. Do as your competitors do.
Oh, and the author name? Keep it unobtrusive, unless and until people start buying books simply because you wrote them.
The best way to tell if your idea for the cover is good, is to look at the top 100 sellers that your book will be competing with. What do those covers say, and how? Be a little, but not much, different.