If JJ Abrams were to write a mystery novel, we could expect complex characters. Given his track record in creating engaging characters in his shows and movies, the novel would likely have a diverse cast with their own secrets. For example, like the characters in 'Lost' who all had hidden pasts.
A JJ Abrams mystery novel would probably have a strong sense of atmosphere. Just as his movies and shows often have a distinct mood, whether it's the eerie feeling of the 'Cloverfield' movies or the sense of adventure in 'Star Trek'. In a mystery novel, this could translate to a dark and brooding tone if it's a noir - style mystery, or an exciting and suspenseful one if it's more of an action - mystery. His ability to build suspense would also be a key factor. He would likely keep readers on the edge of their seats, gradually revealing clues and building tension until the big reveal at the end.