I've always been a fan of murder mysteries - at least in the form of TV and movies, not so much as books. Between watching "Shetland" Season 1, "Hinterland" Season 1, and a talk with a friend, I've come to recognize at least a part of what makes me like or dislike detective shows. This can be stated simply: I want to see them detecting, collecting clues, analysing, reaching conclusions. I don't want to see them in gunfights or fist-fights, or dealing with their own personal issues. But of course it's not that simple: I'm a big fan of "Elementary," which is very much about its two leads (modern Holmes and Watson in New York) - but it declared that from episode 1 of season 1, so it never misled us. "Hinterland" was misleading: the entire first season, we got slight glimpses into our protagonist's life (living alone in a caravan, jogging every day, picture of two young girls he never sees), but not a word is said about it in six hours of media time. But the second season becomes more and more personal, with the main character's messed-up personal life coming into the spotlight, and a lot of lingering issues from past cases piling onto him.
I was a big fan of the original "Law and Order" series: it was one where the police and prosecutors had personalities - but their lives never entered into the story (except maybe when someone left or joined the show). It was all about the case: proving what happened, examining the morality of the legal case. It was often brutally depressing, but man, it made you think. And I guess that's what this is about: thinking. I want my detective shows to be about mystery and thinking - which allows for it to be about the characters of the accused and victim(s), but much less about the character of the detective(s).
There's a measurable problem tied into this - when the police or detectives in the story end up fighting for their lives, it's traumatizing. That's not just a facile phrase: it's a literal reality. People who are shot, or have family and friends threatened, or lose someone through violence - that's very, very hard to deal with. Long-running detective shows that like to involve the lives of their protagonists in the cases they're working on brush this off, it happens to them frequently and they just go on with their work. While the reality is PTSD, psychological evaluations, and career changes to avoid further trauma. This reminds me once again what a great show "Da Vinci's Inquest" was: in the first six seasons, guns are drawn twice - and the only officer who ever fired a gun suffers significant consequences.