I am a natural organiser, and organising things makes me happy. My books were arranged by genre, and alphabetical for fiction, which was very practical so I decided to be completely unpractical and arrange two of my shelves by colour, to create a kind of rainbow:
It's completely impractical, because colour is not exactly a defining feature of a book, but I'm pleased nonetheless. It'll force me to look through things properly when I'm looking for a book rather than just choosing a genre. My other two shelves do look a bit drab now though!