Just saw this thread pop-up, and I'm actually in the middle of rewatching it right now. Halfway through Season 4. I haven't watched it since ten years ago, so its exciting because I've forgotten most of the mid-level plot points. It's even better than I remembered.
I really enjoy how each season is like it's own thesis statement about what's wrong with the system and how Simon/Burns would fix it.
Season 1 - drug war tactics
Season 2 - destruction of working class caused by unions/outsourcing jobs
Season 3 - Hamsterdam - drug legalization
Season 4 - Education reform, get the corner kids out of the classrooms
Season 5 - i'll remember soon enough (failing news media?)
For each argument they show the positives, but they show all the downsides and how the system will inevitably screw it all up.
Also, one tidbit I liked was in Season 3 the detectives were having a discussion about "what real po-lice are still out there doing the good work". One character said something like, "Oh yea I heard Ed Burns out in Baltimore County is really good". Gotta love the flex to tout your own resume on your own show. Reminds me of the Chappelle skit where he writes/directs his own bio-pic.