One of the things that should set science fiction, fantasy and paranormal (SFFP) books apart is their cover. I often see SFFP covers that don't give any indication of the SFFP content inside. So how DOES a cover say SFFP?
First and foremost a cover should reflect, evoke or allude to the content inside, but that in itself is not enough. The cover also needs to include elements that reflect their particular genre. This is especially important for indie authors and/or new authors. Not everyone is Philip K Dick so not everyone can rely on such a minimalist cover to sell their book.
The above cover could be almost any genre, including a math textbook.
Here are some elements that help evoke genre.
Stars and space - mainly they work for science fiction/space travel but they can also work in some fantasy or paranormal novels
Mist - a mainstay of urban paranormal, also might have a place in high fantasy or sci-fiction
Flares - can be used in any of these genres. They are particularly useful in highlighting the title treatment but can also take the place of fairies or other magical creatures.
Magical creatures - obviously only relevant if the BOOK has magical creatures!
Manipulated coloring - can work on any SFFP genre.
Leather! - a mainstay of the Urban Fantasy or Urban Paranormal genre (see also above cover!)
Specialized weapons - relevant to all SFFP genres
For indie authors, the final consideration is whether a cover look can be replicated. Most of us rely on series and serials, companion books and free shorts or novella to help drive traffic to our new books. For his reason we need to make sure the cover design we choose will fit into a strong series concept. Several of these above are part of series. Here are a couple more: