Outlander is basically a traditional romance on steroids. Not only is the book huge, but it takes all the typical romantic elements to the extreme. Claire isn't just your typical fish-out-of-water heroine; she's in a completely different time period than she's used to (thanks to time travel courtesy of the fantasy genre). Jamie is the epitome of the chivalrous gentleman, consistently risking his life for her, and there are tons of smoking hot sex scenes.
But Diana Gabaldon isn't content to just write a romance. She explores the genre, and while she doesn't quite turn it on its head, she's a little more introspective about it than many authors. The scene in which Claire, an independent, strong-willed nurse from the 20th century, is whipped by her Scottish husband, is pretty controversial. How can she submit to that? Is it anti-feminist or is it romance? Scenes like these make us wonder what romance truly means.