Yup, Once Upon a Time. I held on as long as I could, but the Peter Pan thing dragged on and on, and I just gave up. By that point I'm pretty sure every character was each other's half uncle.
Also, Sleepy Hollow. It was alright up until the most recent season when it felt like they were scraping to continue the show with rehashed ideas, and too many haircuts, so I gave up a couple of episodes in.
I'm sure I speak for a lot of people when I say The Simpsons. I still love the earlier seasons to death, but the last 10-15 years are so embarrassing that it actively pisses me off to see the steaming pile of shit it has become.
Not sure it counts, but Defiance. I wouldn't say I ever really got into it, but I stuck it out for the entire first season hoping it would improve, but when I was disappointed that the main character got brought back form the dead in the season finale, I figured it was time to just my losses.