Unique libraries. Like in the Archived, by Victoria Schwab - library of living, breathing Memories, and Lirael, by Garth Nix - a library of dangerous creatures.
Horses. I'll pretty much read anything where horses play a major role. Or even a minor role. Kid stories or adult stories. My all time favorite is the My Friend Flicka series by Mary O'Hara, followed closely by The Horse and His Boy, by C.S. Lewis (I'm reading it to my kids right now). But I love newer horse books too, like the Horse Whisperer, and Sara Gruen's Riding Lessons and Flying Changes, and The Hearts of Horses by Molly Gloss.
Uncommon mythical creatures. Like onis and griffins, especially when combined with the unexpected like Japanese steampunk in Stormdancer, by Jay Kristoff. Or creatures like gargoyles! I want a YA gargoyles-come-to-life book! I actually had the great fortune of beta-reading such a book, and lament no publisher has picked it up yet! (that's for you K.T., still hoping).
Fairytale or myth re-telling. These are hard to pull off - for instance I adored Robin McKinley's Beauty (Beauty and the Beast retelling) and the Goose Girl, by Shannon Hale, but wasn't crazy about Cinder or The Book of Wonders. But they are still instant draws for me - even lesser known fairytales or myths. A retelling of Arabian Nights made me buy the Book of Wonders, plus, my goodness just look at this cover! It was a little to young for me, anyone know of a YA retelling of Arabian Nights? Want. Need.
Historical fantasy. Instant sell for me. Like you wouldn't believe of how instant. Grave Mercy, by Robin La Fevers; Wildwood Dancing, by Juliette Marillier, and best of all, the Temeraire series (yes, mentioning this one TWICE!!!) by Naomi Novik.
Mind or memory twists. Like the science fiction scenario of two minds in one body in What's Left of Me, by Kat Zhang, or carrying another person's memories (I know I've read this but can't think of an example! Help!) Not enough books in this category!Any wicked cool science fiction premise. Like the living prison Incarceron, by Catherine Fischer, or the ability to foresee the path of a choice in Pivot Point, by Kasie West.
By the way, I'm giving away a copy of Pivot Point tomorrow through May 7th so come back!