I realized a long while back I'm not into South Park anymore, which is a shame, because two or so years ago, it was my absolute favorite show/cartoon/anything. I had hats of the characters, I was saving money to buy the DVDs, and every Wednesday there was a new episode, my friend would either sleep over and watch it with me or come to my house the day afterwards and watch the recording.
Over the time, I've discovered a lot more cartoons and my sense of quality/taste has changed. What I used to think was hilarious in South Park now only causes me to chuckle, if that.
There are still good episodes of course, like ImaginationLand, most of the Season 7 to 9 episodes, but I can't find myself re-watching them and enjoying them as much as I used to (Especially since I know all the jokes at this point). I'm probably a season or two behind on the series.
Same thing happened with American Dad, except I wasn't as into it as South Park.
Oddly enough, shows that haven't had any new material, such as Invader ZIM, I've gotten tired of, but Xiaolin Showdown has no new material, and I've been of fan of that show for nearly 5 years now. In fact, I got more into it when I re-watched most of the series recently (My interest didn't grow back when I re-watched Invader ZIM that much).