Main problem was they got rid of Doakes.... big mistake.
Seasons 1 and 2 were great because Dexter had to: please his killing urges, maintain his cover with Deb and the cops, balance a homelife with Rita and her kids, kill all of his enemies and while doing all of that have to duck and dodge Doakes the one person that sees through all of his cover-ups and smoke screens. Made interesting television.
Once they got rid of Doakes it was just too easy for Dexter and he was on cruise control for the rest of the seasons. Doakes being a constant thorn in Dexter's side and being the one that would eventually expose him was something that would have kept the viewers watching and interested... but nope they just deaded all of that. To be fair you can look back and clearly see that Showtime or the creators of Dexter really didn't know that a show about a deeply disturbed serial killer would become so popular so they used all of their good ideas on the first 2 seasons.
Also another main problem was that as time went on they tried to force attention to other characters that we didn't give a shyt about. Nobody cared about Deb and her problems, Angel's dating life, Quinn in general. The asian dude was the only other person that had personality since he was real life Quagmire with his love of women and sex.... sadly they rarely showed any women he was fukking.
Another reason was that Dexter is a prime example of a show being too damn long... should have been only 5 seasons really and it should have been like:
Season 1 - same as how season 1 originally was with the Ice Truck Killer Arc... don't change anything from this.
Season 2 - Have the religious theme from season 6 with the Doomsday Killer be the main arc with Deb finding out about Dexter's serial killer life
Season 3 - Trinity Killer Arc with Deb coming to terms with what Dexter is and the introduction of Special Agent Lundy
Season 4 - Dexter dealing with the loss of Rita, deals with Dr. Vogal and how Harry's Code was created for him, plus backstory of Dexter's dad, plus side story with Isaac Sirko and the Ukrainian Mob
Season 5 - everything Season 2 was with the Manhunt of the Bay Harbor Butcher Arc, Doakes locked in the cabin and Dexter at the end denouces Harry's Code for his own code