the Matt/Ronnie relationship
I really think they're the emotional heart of the show. I've been working on re-watching the original eps that these are based on and it feels SO different. Bad Faith (that Confession is based on) was just so flat because Briscoe and Logan don't have half the relationship that Ronnie and Matt do. Logan didn't even TELL Briscoe that the priest was an abuser for a long time.
It really all comes down to the kind of chemistry Ronnie and Sam have. Which is why I'm DYING to find out who is playing Sam. I'm guessing publicizing the new character is a bit of a spoiler and that's why they haven't done it? Because otherwise they really should be getting as much press on the new regular as possible.