When I'm filtering by moods or genres, I'd really love it if there was a way to say that one filter is required while others are optional. For example, making adventurous required, while making lighthearted and mysterious optional would bring up books that were adventurous and mysterious, adventurous and lighthearted, or all three, but no books that were just mysterious or lighthearted.
Or for genres, being able to select romance as required, but historical or fantasy as optional to bring up historical or fantasy romance, but not contemporary romance or non-romance fantasy.