I think it would be really helpful to be able to choose more than one genre for a book. Being able to pick three, or at LEAST two, would really help with describing one’s book and getting it in front of the readers who could be interested in it.
My book,
Fae Child, is a YA Fantasy - but I can only pick one or the other at the moment, which halves my audience.
I’ll definitely second this. Especially considering that some of the genres we have available are actually subgenres. Tantalus Depths is listed under Hard Sci-Fi, for example, but I don’t think it would currently show up if you were just searching under "sci-fi." The same would go for the Space Opera subgenre, or for the Fairy Tale subgenre of the Fantasy genre.
Not to mention that fantasy, sci-fi, realistic fiction, historical fiction, biography, etc. could all, depending on the nature of the story, also fit under the genre of humor, adventure, thriller, etc.
Almost every book on this site could potentially be considered two or more different genres.
My book, Fae Child, is a YA Fantasy - but I can only pick one or the other at the moment, which halves my audience.