Action. Romance. Mystery. Adventure. Suspense. Literary. Fantasy. [your preferance here]
The way I see it, one genre is like ice cream -- good, but limiting. Many genres is like a Thanksgiving feast -- wonderful flavors to satisfy all the different tiny cravings an eater can have. And so many layers to delve into and explore. So many elements to intrigue and delight.
Of course, all published books today must have one dominant genre, otherwise bookstores don't know where to shelve them! However I despair of romances that are only about the emotions of falling in love, and the thrillers that are only about the chase. Give me complexity of interest and both relationships AND adventure!
This was one thing I enjoyed about "Inception." It broke genres all over the place, tying in so many different themes. It was adventure and heist and loves story and psychological thriller all rolled into one.
Or take Harry Potter. It's a fantasy adventure that is also mystery, political comedy, and even shades of Jane Austen (Rowlings claims "Emma" as her favorite book).
My writing always bears multiple flavors. The hard part is choosing the dominant one...