A lot of people's perception of the original-cast SNL days is based on the syndicated packages and best-of compilations that only take the high points. I bought and watched the complete sets of the original five seasons, and... boy there was a lot of crap mixed in with the classic stuff. The good stuff definitely outweighed the bad, especially after the early days when the show was finding itself... but it definitely wasn't the "home run every at-bat" show that some people think it was.
Looking back, only a handful of SNL's 46 seasons have been truly awful. Seems like a rule that the show is at is funniest in the second half of each decade, but it's actually pretty good now.