Re: Long-running TV shows you invested seasons in but stopped watching
I stopped watching House in season 6, though it wasn't really a conscious decision. Somewhere along the line, despite loving the premiere and really enjoying "The Tyrant," I realized I had fallen behind on episodes due to other shows and real commitments, and I haven't taken the time yet to get caught up. I probably will at some point considering I own the first five seasons.
Castle I stopped following after the end of season one simply because, as good as the cast chemistry was and as fun as it is to watch Nathan Fillion, I just lose interest in most procedural shows.
The Dead Zone was dropped from my schedule at some point during its fourth season after "Heroes and Demons." I had loved the first season and still enjoyed the second season, but season three and especially four took a sharp dive down in quality so I lost patience for it.
I dropped Heroes during season three, but I had never been entirely onboard with it despite some promise in season one. Everything after that, though, was downhill, and I didn't even think that highly of season one in the first place.
I'm considering dropping Lie to Me, currently in its third season, from my schedule. The show is pretty enjoyable and Tim Roth is fantastic, but it's doing a pisspoor job of developing any of its supporting cast and that's rather annoying. I came into the show during its second season because Shawn Ryan was brought in as showrunner, but he departed after that season to work on The Chicago Code and, although Alexander Cary doesn't seem to be a bad showrunner, his absence is noticeable.
There are probably other shows that I've dropped over the years, but those are the only examples that stand out to me right now.