Growth
I love growth. Emotional growth is my favorite, but creating a loving relationship with someone is a close second. And if my heroine is a kick-ass hero at the end of the book -- she isn't that way at the start.
Really! When you are done with my book, I hope you start over (because it's such a great book) and are shocked by how different the heroine is – Because I changed her so slowly you didn't notice. Um, that's hard to find in any book.
The growth and change is never mysterious -- something happens, and she learns from it. The falling in love also makes sense.