One of the first things was how thoughtful he is. He'll hold doors, he remembers little things (although he can be forgetful, too; one day I'll figure out how he stores info), he'd bring me small things, like a dark chocolate Hershey's bar because he saw it when he stopped for gas. That sort of thing. And he listens. My mom has been putting the family through eHell for the last year, and Chris has always been willing to lend a sympathetic ear, no matter how many times I've needed to go over the same detail to work it out in my head.
He's also so good with my family. He found out that my sister was failing math, insisted on helping her, and got her up to a solid B, without ever losing patience. He deliberately wears his favorite football team's jersey when we see my dad, because he knows my dad LOVES teasing him about it. He listens to all my grandpa's stories, even when it's one he's already heard. When my grandma was sick, he found her the perfect get-well card, totally unprompted.
On top of all of that, he makes me laugh even when I just want to sulk. It's never in a patronizing way, he just always knows what to say to snap me out of whatever problem I've been dwelling over.