The only thing people will care about tomorrow morning is the kiss, so I won't bother talking about the plot other than that they now have an assassin who was hired to kill the dirty former detective. I guess you could call the kiss a fake out since it did not occur under normal circumstances. On the other hand, it was definitely a conscious decision since we see Castle stop Beckett from drawing her gun, and both are really into it. What will make or break the moment is what the writers do with the fallout in the next episode. If the writers follow up it and make something constructive out of it (not more artificial roadblocks and postponing), then it'll be worth it. If, instead, the writers ignore it or allow this thing to drag on longer, then we can collectively roll our eyes because everyone knows how they feel about each other.
As for the rest of "Knockdown," I liked the change of pace and tone, which is necessary for the subject matter, and the acting definitely fit in line. It'll be a while until we do find out who killed Beckett's mother, but these episodes are very good so far.