^^ I think either Moonlighting or Cheers started it all. In this case, the 'will they/won't they" takes a back seat to just how fun this show is (at least for me). I rooted for Sam/Diane, and David/Maddie, and Jim/Pam, but I don't care that much about Castle/Beckett. I'd be ok if Beckett just up and said "no, we're not doing this" and then they went on from there.
Fat chance.. there were several scenes in season 3 heavily implying she's also fallen for Castle but as he said when he visited her appartment she's been emotionally closed off since her mother's murder and won't allow anyone to reach her.
However Castle seems to be the first person to really achieve that as evident be her slight smile when he finally said it and she passed out (of course she passed out.. they can't kill off a main character when the show got renewed).