As others have said, Patrick and Cosmo have suffered from this lately. Patrick was dumb, of course, but I never remember him being so dumb it was annoying. He was aware of his surroundings, but his ideas and thoughts were what made his character. Now, he seems to have lost any and all common sense...The rest of the cast of SpongeBob seems to have become one-note, too. SpongeBob was much more intelligent with occasionally strange, but mostly naive and over-reactive, actions. Now, he acts too over the top and stupid. Squidward exists almost solely to be annoyed, Sandy is nothing more than a scientist with wacky inventions (which really strays from her original character), etc, etc. The same thing happened to Fairly OddParents.
I suppose characters are changed because they're easy to write and kids still watch. Or something. But it takes away from the original personality and obviously there are teens and adults who can easily recognize the changes. Even the kids, when they are older and watch the shows they grew up with, will notice the lack of rounded characters.