I dunno, I'm getting tired of shows where the case of the week just happens to resonate perfectly with whatever's going on in the main characters' lives at that moment. As soon as they begin a secret workplace romance, they get a case involving a secret workplace romance? It's too contrived. Not to mention that they did a pretty crummy job hiding their reactions. Anyone watching them during their interrogations of a couple of witnesses would've caught on right away that they were sleeping together.
Also, what's with the sudden retcon of their reasons for keeping it secret? Last week, it was Castle wanting to keep the secret for rather nebulous personal reasons and Beckett who needed to be convinced. This week, it's Beckett who's worried they'll have to stop working together if their relationship becomes known. Now, the latter explanation works much better as a justification for the secrecy, but why couldn't they go back and rewrite the last episode to incorporate it? After all, usually the first several episodes of a season, or even the whole first half or so, get their stories broken (outlined/worked out) together before shooting begins, so they should've had this idea in time to go back and incorporate it into the previous script. So it's an odd inconsistency.
My favorite bit was where the suspect lawyered up first thing and refused to talk, so Beckett and Castle put on a show of going over the incriminating evidence in order to provoke him to talk. Admittedly it's shaky from a procedural standpoint, but it was a clever variation on the usual interrogation-scene formula.
Although they're still doing their usual formulaic thing that bugs me, where they interrogate a suspect they think is guilty for several minutes, and then finally they ask for the suspect's alibi and it turns out to be ironclad. Why don't they just start with the alibi question and save themselves a lot of trouble? I know that would make the episodes a lot shorter, but it makes the cops look silly when they expend so much effort pursuing someone that they could've cleared already if they'd checked something as basic as their alibi earlier on.