Agents of SHIELD - I've heard it's gotten better but S1 was painful to sit through and I got maybe 8 or 9 episodes in. As much as I like their films and the netflix shows, I just don't have that interest for a show focusing on SHIELD and what their agents are up to.
The Simpsons - Used to be really fun, but not just that, it also had heart and was a well written show. Later seasons are mostly filled with product placement, references to modern culture and it's lacks the comedy and heart that made it so great initially.
Heroes - First season was really great, and I remember following the show even if declined, but the last season could just not keep my interest up. I was kinda losing interest before that, but for whatever reason I stayed longer than I should have. Never watched the re-launch.
Fringe - A nice little show, but to me it felt like a less interesting X-Files. I watched the first season but didn't feel a strong urge to return.
The Big Bang Theory - Loved the first few seasons, but as the show has gone on, it's not really funny anymore. What used to be great character perks are now annoying and overused, and I really hate it when they take something that was fairly subtle and subdued in the first few seasons (and worked because of those reasons) and turn it up to eleven and it loses everything that made it great in the first place.
Lastly, while I haven't left the ship yet, I feel like I should jump ship on The Walking Dead. I know a lot of people who paint it up like one of the best shows on television, but it is painfully mediocre. I liked the first season, but every season since has been a chore to get through. There are a few good episodes per season, but far too few to warrant sticking to it. At this point I feel like I'm watching for the wrong reasons. Getting some form of closure after all the wasted time, getting amusement from "so-bad-its-good" moments, and just sticking with it so that people can't shove the whole "the show is much better now, I promise" argument up my face.