Shows that improved after the first season
I think that usually the first season is the best or one of the best seasons of a show. It still feels fresh, it has a clear concept and very often they feel the need to change things around in the second season (and not always for the better). Sitcoms may be the exception, because comedy is a complicated thing.
But what are shows that significantly improved after the first season?
EDIT: I'm going with a random one. Matlock!