Inspired by Caj and his thread about characters introduced late in a show's run that worked.
In this case, characters/ actors that left and the show just was not as good anymore.
The first one I think of is the Andy Griffith show when Barney Fife/ Don Knotts left.
In fairness, two of it's three seasons as the number one show were after Barney left, although one of those can be explained by the viewers knowing it was the last season.
But most people, including AG himself, mark Barney leaving as the point where it went from having something special to becoming just another sitcom.