Jericho. OMFG, Jericho. A guy at my work really got into it, so I gave it a try. I made it about 5 episodes and had to stop. There are only so many inexcusably poor decisions I can watch people make unless it's very funny. Jericho wasn't funny, it was just sad and stupid.
The only other show I hated so much I quit was The Office (British version). I quit that after just one episode. My thoughts went, "OK, this guy is a massive tool...okay, he's still a tool...okay, STILL a tool...still waiting for this to be funny...what? Roll credits? Are you fucking kidding me? THIS is the show everyone's raving about??"
Often enough I will watch 2-3 episodes of a show and say, "Nope." This isn't necessarily because I get pissed off, I just don't care about anything that's happening. The Sopranos was like that. I know, I know, highly rated. I found Tony unlikeable on any level.
There are several entries in the category of Shows I Watched All The Way Through And Wish I Hadn't
Lost. You got the idea early on in this show that the Island and its mysteries were Things Man Was Not Meant To Know. Then you discover the whole plotline of season six was Things Man Was Not Meant To Write. Everything that happened was an obvious ass pull, every explanation was lame and unimaginative, etc. The wine cork analogy from season two or three was cool -- until we found out that the Island's evil was contained by a literal cork, a plug that can be pulled out as if from God's own bathtub? What the absolute fuck?? I could go on.
Battlestar. I really got into the first season. Then the show started a long, slow descent that reached its nadir at the final episode. Yeah, let's take all our miraculous future technology that makes life bearable -- and pitch it into the sun! Now let's interbreed with African pygmies and forget everything we ever knew, so we'll have to spend 25,000 years discovering it all over again! Total idiocy. Honestly, was there anyone who thought this was a good ending?
Heroes. Dynamite concept, well executed for a season or two. Then it just started meandering. For me the jumping-the-shark moment was when Mohinder turned himself into Spider-Man. I had to keep watching, but it was clear the show had lost it.
Oz. Tremendous first couple of seasons. Season 5 was like a fever dream, the show taking a sharp turn toward the supernatural. Odd and, given that the show had been thoroughly grounded in reality up to then, totally unwarranted.