Alien superpowers, and nice cheekbones.
I'm sure everyone who finds Spock attractive has their owns reasons. But there are so many great reasons to choose from. ^_^
Certainly, I recognize that Shatner had more of the classic hero-type good looks, but again, not everyone is attracted to that type of character. And Kirk just seems like such a player that that's a turnoff for me in itself. You just know he'd be checking out all the green orion slavegirls. Spock wouldn't even notice the dancing girls.