The title must be readable and attractive. Characters and situations must be accurate. Period.
Growing up, my siblings and I would bemoan some book covers merely because the blond heroine usually ended up being a brunette or a redhead. The books that had such disparaging illustrations were avoided on that basis, unless the story had some powerful element that overcame our dislike of the illustrations. As I grew older, I realized that many other boys and girls felt the same way. Now when I illustrate a story, I will read the story completely and will often list details from the story for later in the illustration process.
Clarity is a catch-all for the details that signal a finished illustration. Not only should an illustration clearly illustrate the story, but the title should stand out clearly on a cover illustration. The illustration should not look awkward with type. Readers should recognize the important characters immediately. The texture, or lack thereof, should not distract from the illustration. The detail should be well-balanced throughout the picture. Sometimes a change in value or the addition of subtle outlines helps make a picture more clear and finished.
Of course exceptions happen, or time requires a less thorough attention to detail. However, most illustrations should exhibit this clarity.