I liked the implant/boob joke by Beckett.. shows how far they've come in their relationship. Back in season 1 she wouldn't dream of mentioning anything boob related to Castle
Very nice case.. something unusual with a tragic ending that sets it apart from the other cases.
Also liked Molly this episode.. growing up is hard to do and there are tough lessons for all of us and now it's Molly's turn. Castle handled it very well showing that despite all his antics and teenage actions he's still mature and an adult when the situation calls for it. It was clear Beckett wouldn't die in the season finale much as it was clear that Molly wouldn't leave the show but nevertheless the writers managed to make it believable and get a good character moment out of it.
As to the Beckett/Castle relationship.. i think the writers (and Marlowe) are really worried to take that final step. They do want to as well as the actors judging from the interviews i saw/read but they are taking it slow. They took a big leap in the season finale and have found a somewhat believable way to prolong the inevitable just a bit more but if by the end of the season nothing major has happened i think they'll tread in very thin ice (and that's not only the shipper in me) because you just can't keep throwing us longing looks and innuendos for multiple seasons without it losing its appeal.