My wife and I were talking about long-running TV series the other day, and I couldn't help but think of The Fairly Oddparents. I liked that show when I was younger. I haven't seen it in years, but it seems to me that it went through several distinct phases.
Seasons 1-3 were really creative and inventive, coming up with neat ideas based on the premise of magical fairies who can grant every wish. It was good stuff.
Seasons 4-5 were not so good, as the show became more of a generic sitcom. Creative premises were replaced by more typical premises, like "let's do an episode based off a movie" and "celebrity guest star episode". Also a lot of "reuse a previous idea/character that was popular", which was hit or miss. That was when I stopped watching the show, and it was cancelled.
The show was uncancelled two years later. Seasons 6 and 7 introduced Poof, the new main character. I don't much care for him, myself, but I enjoyed those seasons. The show went full-blown comedy at that point, and I liked that. Off-the-wall comedy works well for the show.
Season 8 was a strange one, with only six episodes. They made each episode double-length. The show was basically the same, but longer episodes made it different enough, to keep it fresh.
Seasons 9 and 10 tried to shake things up, with a new main character each season. I haven't seen Season 10, but Season 9's new main character was kind of a failure. He was a weird Rick Moranis parody, and I'm told he hasn't been seen since.