The later episodes seem more dated...
Is it funny but watching the show- the earlier seasons seem more timeless in their delivery- even though you can tell its from the early 70s. Maybe the episodes are less trendy in a sense, if you know what I mean. The later episodes seem like the show became too "aware" of itself and the need to be cutting edge. This happened with other shows like "Happy Days" (hence the term "jump the shark)
Mary aged considerably in just a short amount of time. The humor became more dry and cynical. Mary's sweet personality was lost. Mary would never make a joke about Suanne Nivens or something. It just seems like she changed. I guess she seemed jaded in a way. Probably because was over 35 and still not married with children.