The most important thing is to look at books in your genre, see the trends and attempt to emulate them. You want to stand out, yes, but you also want to make it crystal clear for your readers what kind of book this is. And making sure to look like the other books of your genre is a big part of it.
Nice idea when you write in genres big enough to have standard ways of making cover art.
This train of thought began, actually, when I showed my proposed cover art for my historical crime novel to people in a Georgian/Regency forum. Everyone hated it, mostly becuse they
couldn't seem to wrap their heads around that
it wasn't supposed to be a romance novel and they kept telling me to make it look like a romance novel (which would most certainly lead to disappointment of anyone who bought the book expecting a romance novel, because it isn't one.) The photo I found was a nice pulpy-looking scene of a screaming woman in 18th century clothing, apparently being attacked by a gentleman in matching dress. Since it didn't go over well with the historical crowd, and the costumes in the photo would probably be off-putting to the crime crowd, I am contemplating going more abstract with an artsy painting or drawing, but that might just put people off in general.