So, my cover artist and I went to the range recently. On the way back, we found the time to tackle things even more difficult than shot placement and proper grip: cover art. She'd sent a mockup of what would be a great cover for someone else's book, and I had to think for a few days about why I didn't like it before I had an answer.
Me: "The problem with the cover is that it clearly conveys military scifi, but this book isn't modern MilSF; it's an homage to Andre Norton, Leigh Brackett, Lovecraft and Jack Vance and Scientifiction. Back before the genres were near as split as they are today, and you could have psychic powers and fantastic alien ruins of unknown races and remnants of the eldritch... If modern readers pick it up expecting a MilSF full of modern tropes, they're going to be unhappy. But how do we signal a pulpy retro Astounding and Weird Tales vibe?"
My cover artist: "Challenge. Accepted."