Ignoring genre signals
Thrillers, romances and fantasy novels each have a visual language. Break it and readers will not know what your book is. Stand out inside the genre, never outside it.
Unreadable titles at thumbnail size
Most readers first see your cover as a tiny image. If the title cannot be read at the size of a postage stamp, it needs larger type and more contrast.
Too many fonts and effects
Two typefaces are almost always enough. Heavy shadows and glows tend to look dated and cheap.
Low-resolution or stock-photo clichés
Print needs 300 DPI. Overused stock images make a book look like a template, so pick distinctive imagery or commission original art.
Forgetting the spine and back cover
Paperback buyers flip the book over. A clear tagline, a short blurb, and a clean barcode area make the whole package feel professional.
Skipping a series look
If you are writing more than one book, plan a consistent design system from book one so readers recognize your work on the shelf.
Not testing before launch
Show two or three options to readers in your target audience. Their first reaction tells you more than any opinion in your own head.