I had only seen season 1-4 of Dexter, until they popped up on Netflix. I just finished season 7.
SPOILERS!
What happened? Did they change writers? Dexter went from being an anti-hero who killed bad guys that slipped through the cracks, and suddenly became a villain by leading the police in the wrong direction so he could take the law into his own hands. Also, I'm no crime scene investigator or lawyer, but it also seems the legal aspects of handling evidence and proving guilt have slipped. And don't get me started on the whole Deb being in love with him. I'm wondering if I even want to watch the last season, but might as well see how it all ends.
As I understand it, yeah, after Season 4, the original showrunner left. This explains the dramatic drop in quality moving forward.
While some of the stories in the later seasons were interesting and entertaining, there was a pervasive sense that nobody had any idea where they were headed, and they were simply making it up as they went along. I don't recall specifics, but season 5 had several plotlines that were just...dropped. The sixth season time-jump also threw me for a loop.
There were two things that really irritated me about the show, which gradually grew worse and worse, and which were true even as recently as Season 2.
1. The writers never seemed to know what to do with Dexter himself. He'd spend an entire season trying to accomplish something, only for them to hit the reset button at the end of the season. He'd get close to people, think maybe he could find redemption, then >bam< they'd leave or turn psycho, or he'd run from them or whatever. It just got old. Really, really old. It made each season seem more and more pointless when, at the end of the seasons, they'd bring him back almost exactly right where he started. Season 5 and Season 6 seemed, to me, to be the worst offenders here. The relationship with Lumen that then abruptly ends with him saying "I guess I'm just a bad person after all." The search for religious redemption, which ends with Brother Mos Def being killed and Dexter saying "Nah, never mind. I'm just gonna kill people." Why go down these dead ends if they just end up with Dexter reverting to form each time? He never seemed to grow as a result of it, either.
2. The whole "will he get caught" thing just got really stale. The notion of a serial killer balancing his "real" life was intriguing, but it got to the point where it was just frustrating at how they'd try to create tension by having Dexter constantly being two seconds away from getting caught. It just made me roll my eyes when they were STILL going back to that over and over and over.
In my opinion, the first season of Dexter is one of the best seasons of TV out there. But everything after that can largely be ignored. The 4th season was pretty good, but even that had its flaws.
