They say "don't judge a book by its cover," but really from a marketing standpoint, that's an egregious dereliction of duty - a wholesale abdication of what a cover is supposed to be for!
A cover's whole point is to equip prospective serious book-browsers with sufficient cues from which they can judge the book. And if the cover does its job, judge it worthy of further investment. Time, for reading. Money for purchasing, possibly - unless the browser is one of those pinchpennies who haunt libraries on the lookout for a cheap read. But the point is: the cover is key. Without that lure, those precious contents aren't going to be getting that loving read they crave. So better believe that cover is important! Put some oomph on it, and give people something to judge by.
A well-designed cover presents an accurate and appealing idea of the contents. It selectively attracts those who would be disposed to enjoy those contents. It entices the target audience. Every aspect of the cover is calculated to single out and persuade that desired audience. You're not appealing to everyone in general - different people want different books, and are going to feel misled by a cover that woos them for something the book can't back up! There's no value to a misleading cover. You want to make your bold best pitch based on what you're actually selling. Beyond that, it's wide open! Get creative, but stay on-message.
The back cover: are there blurbs and quotes to persuade and cajole? Is there a plot tease, to rope in those who may not be familiar with this author's works, but who could be hooked in by a story synopsis that sounds right up their alley? Does the entire back cover consist of a black and white author portrait? This can broadcast a number of messages. "Hm, perhaps this author is too important and established to worry about blurbs and plot points. Maybe I missed out on this person, and need to check them out!" "Hm, this author looks smart enough to engage and entertain, while not being so sexually attractive as to be unrelatable to me the reader." "Wow, this person looks weird and intense, like I like people to see me!"
Think carefully about what's on your back cover - you can't fit everything! So what is on there has to be calculated for not just maximum effect, but for the correct effect.
The front cover, all the more important. Often the front cover you present is what first catches the eye and draws the potentially serious browser towards you. Is the title clear and eye-catching? Is the font right for the image and tone you're trying to project? Is the whole cover just a garish assemblage of random bright-colored objects on a black background with neon lightning flashing over an anonymous metro skyline? Who are you trying to attract, and what are you trying to tell them about what's inside?
I guess what I'm trying to say is: tuck your fucking shirt in, ya slob.