The fact that Spock's not supposed to be sexy is what does it for me. There he is all prim with his utilitarian haircut and intimidating intelligence, and you just know he's an animal underneath.
He's a challenge-- what must you do to break that stoic exterior? Granted, nuSpock flipped his lid pretty easily, but under normal circumstances (normal being not a day during which his planet was blown up), I assume he's enticingly resistant to any sort of emotional goading.
Eh, I'm a weird girl, what can I say.