I think it's probably the kiss for me, not necessarily because the Doctor looks slightly surprised, but more Rose's side of things. To me it's such an "I am FINALLY going for this" kiss. Yeah, you could say it was just because she finally heard those words, but you could just as easily go the other way.

Eh, maybe it's all related to the reason why I would flop back and forth in the first place - it never seemed like sex was necessary in their relationship to show their love to each other. I think they were heading towards it (it being a progressive thing as opposed to something needed to cement their relationship), and they're most definitely are having sex in the altverse, but... Personal canon, I guess. :D