Just have to chime in again to agree with your assessment of Season 3. Although I haven't seen every episode I, too, feel that this is the first really solid season. And while I do agree that the show overall got noticeably harsher as time wore on I also have a hard time penetrating many of the references from the earlier seasons.

That said, the longevity of the show has been greatly enhanced for me because as I get older and my nerd horizons expand I get new jokes. So maybe one day I'll be able to pick out gems from the first three seasons that I don't get now....