One of the biggest mistakes I've seen-- usually in authors who are so happy they don't have to have offensive genre covers anymore-- is that they forget that covers are supposed to convey genre. You don't put sausage in a cereal box-- even when it is a pretty cereal box.
Also, and I don't know how you teach this (anymore than how you teach good story telling) a cover needs to catch the eye, then hold the attention long enough to convey what the book is about and hopefully get people to have a deeper look. Many images are worth a first glance but that's it. There needs to be some extra little detail that catches the eye-- a single shoe on a kitchen floor, a dragon tail poking out under the sofa.
Lastly, if the book is part of series, there needs to be a unifying theme. Perhaps the same font on every book. Maybe each book will have a picture of a calico cat or a dictionary. There needs to be a visual that says this book is part of a series. Readers can forget names and forget titles. Usually visuals stay with them.