Well, I have to say I that finale had me on the edge of my seat. It was neat to see Nathan Fillion bringing in a stronger, almost darker side to Castle; it gave me a flashback to Mal's darker moments, the kind that would make a fair amount of his Firefly crew blink, but not Beckett, given their dynamic.
I have to confess, my initial instinct when they started to allude to a cop within the precinct was, "It can't be some random cop, and there is something so huge behind all this, consequently the cop has to been in the highest position possible." And then I freaked out, because my instincts were pointing directly to Montgomery himself. I forced myself to set this aside, and wait and see how the story unfolded from there, if any new characters were introduced or something.
As this finale ran it's course, the same thought popped up again, early on. I was dismayed to see Esposito and Ryan's reaction to the big reveal, I love those guys; their such a great double act and they are the perfect offset to the melodrama of Castle/Beckett. It's nice to see how those four are now a more strongly bound family, by the secret that they keep.
There is great potential to shake up the dynamic in the show, it can really move forward in interesting ways. I agree with people who said it would be such a tedious development to have Beckett have some sort of memory loss or something, to flinch from that breakthrough.