Cougar Town - I watched about half the first season before deciding I couldn't take any more of the unfunniness.
The Venture Bros. - I absolutely loved the first season, then was extremely disappointed with S2 (I think I liked all of 2 episodes), though everyone else in the universe claimed S2 was better than the first. I started watching S3, but it was on the same level as S2, which is great for everyone else, but unfortunately not for me.
Family Guy - I was late to the party, and devoured the pre-cancellation stuff. Got to the first season post revival and loved the first ep, which put me in hog heaven. Then I absolutely hated the rest of that season. I watched the first ep of the next season and thought it was great. Finally, we were back on track! Unfortunately the following eps were just like the previous season, so I pulled the plug for good.