The answer is "a lot of things," of course, but for me, a few of them are: stunning prose, gut-wrenching obstacles to be overcome, incredible characters, and a timeless love story.
Oh. And a beautiful cover doesn't hurt.

I've been in love with
The third and final book came out last week and It. Is. INCREDIBLE.
What? You want some sample lines? Nooooo problem.
"Overhead, the stars were wheeling and infinite, a complicated mobile made by giants."
I know, I know. Now, how about this:
"There is no better taste than this: someone else's laughter in your mouth." (*sigh* *swoon*)
It's these kinds of things that make me fall in love with an author and leave me star-struck by her. I've said it before and I'll say it again: Maggie Stiefvater is a prose goddess in the pantheon of YA authors, and I bow to her.
You know what might be approaching how incredible this series is? The fact that Elana Johnson (another author crush and prose goddess, yes, you'll hear more about her from me later, DON'T WORRY) is giving away a SIGNED ARC of the last book, Forever today on her blog!!!!
If I win it, I won't send it to any of you, but I will let you borrow my Kindle copy. So, that's something.