The finale of Season 5 was poor. Too much screen time was spent interviewing Hastings (I actually dozed off once or twice), who we all knew wasn't going to be the real catch, while the work to nail the actual criminal went on mostly off screen.
But what annoyed me the most was the return to Dot's dying declaration video. While he had a giant fatal wound in his chest, apparently he was tapping out messages in Morse code? Get real. That's so incredibly implausible, and now they're going to build an entire sixth season off of that ridiculous premise.
And all of the non-case stuff like Steve's back injury and Kate's relationship problems didn't go anywhere. I still want to see Season 6, but I felt like Season 5 was the show's low point so far.