Perhaps one of the worst sayings ever thought up is "Don't judge a book by it's cover!". Ok, I understand the real point of the saying but the literal sense no longer applies in the same way it did when book covers were nothing more than picture less hardbacks with an embossed title. These days it's hard to overstate how important a book cover is and how a badly designed one will put off readers like nothing else!
7. Not optimised for print
There are some important differences between eBook covers and print covers and understanding those differences is vital to producing a high quality print cover. The conversion from eBook to Print is not a simple case of dropping the eBook cover onto print template and scaling it up. A professional print cover needs to have higher resolution images that won't become blocky (blurry) when printed. Other potholes include titles and text too close to the edges and poor use of colour.
6. Wrong Aspect Ratio
Fiction books are not square so why should your cover be? Even if your novel is only available on Kindle, it should still have a similar aspect ratio to that of a print book. If your potential readers see a square cover on Amazon, they may think it’s a music CD or and Audio book.
5. Too many fonts
I usually use 1 or 2 fonts, certainly no more than 3! If you use a different font for your title, tagline, author name, series strapline and cover quote it becomes messy and unfocused. The more font types you use, the more the more jarring it becomes to read and also the more contradictory the tone becomes.
4. Too many elements
Just because your story includes a flying motorbike, a drunken Elvis impersonator, a snowboarding monkey and an over-sized portrait of Barbara Streisand, doesn’t mean they all belong on the cover! The more elements you include, the more they will compete with each other and the cover loses its focus. Effective cover design conveys a message by focusing on a single concept.
3. Cheap Tricks
The first one that comes to mind is the 'negative' effect that inverses black and white. Others include 'art' effects like 'coloured pencil' or 'edge trace'. All cheap filters scream "amatuer - avoid".
2. Poor image quality
Both in terms of the standard of the original photograph/illustration and also the condition of the image. I spend a lot of time searching for the highest quality images and edit them in a non-destructive way. That means I don't stretch them out of proportion or anything that will harm the image integrity.
1. Illegible Typeface
It's tempting for authors to want to use a typeface that matches the theme of their book and while some thematic fonts are fine, many are so elaborate that they become difficult or even impossible to read. I've seen cover fonts so bad, I literally could not read the title or subtitle. And the ultimate crime of all is thinking it's a great idea to then use the same typeface for the whole book!
In summary, it doesn't take a designer to spot a bad cover but it does take a designer to create a good one! The standard of your cover is consciously or subconsciously telling someone how much you care about your book, it's that important.