After last season's game-changing cliffhanger, the premiere needed to slow down a bit. We all knew that Beckett wasn't going to die, and it was too early for Castle and Beckett to become a couple. I think the premiere worked well at slowing the action down and setting the stage for the season.
I was looking forward to the appearance of Penny Johnson Jerald as the new precinct captain since she was excellent on 24. She was good, albeit toned down from her 24 days. The whole insistence on being called "sir" was strange, to say the least. I look forward to more episodes with her.
It was fairly obvious that Beckett remembered that Castle said he loved her, I mean, how couldn't she? She'll keep it under her hat for a while, but eventually she will have to tell him. She knows, but Castle doesn't know that she knows. When he finds out, I imagine he'll be pissed that she lied to him. I don't see that happening before the season finale, though. And anyway, he has a secret of his own. Him going off and investigating the death of Beckett's mother on his own will not end well for anyone,. She'll be pissed, and it may be a relationship-ender. I hope not.
Since Castle thinks she doesn't know, he'll probably start dating again. It'll be some dumb yahoo who's in it for the money. Beckett won't like that. Maybe she'll get it out in the open before it snowballs.
Another thing that has ramifications for later episodes, Beckett is experiencing symptoms of PTSD. She froze up in the line of duty, and she started going back to her therapist. I don't know if she'll get over it or not, but I figure Castle might eventually be a major part of her recovery. Depends on if he's mad.
Overall, the mystery was pretty weak, but it was more a character piece for Castle and Beckett.