Game of Thrones:
I stopped watching after... season 4, I think. The show had only been holding on by a thread with me. It's well done... but it's not a story.
When I say it's not a story, I mean it doesn't have anything resembling an arc or theme. Oh, some fans say it does, but they're projecting. It's not actually going anywhere, it's just a perpetually meandering soup of stuff happening, like a soap opera. Characters die, power changes hands, and then it all happens again in a different arrangement, and then again.
And nothing is going to change. It's painfully obvious that however it "ends", whoever gets the throne is gonna spend the likely short rest of their life fending off the next batch of psychopathic social climbers. Same with whoever gets the throne after them. It's in the culture of the world, no war or ruler is going to make a difference. It'll end slowly over centuries of incremental shifts, or it'll end in the medieval post-apocalypse of a second Long Winter (which will reset the cultural clock to "life is hell, people are worse" rather than improve anything). This world is a crapsack hamster wheel and none of these power plays matter.
It's just an asshole contest between sociopaths, and anyone who's not an asshole sociopath exists merely as a stage prop for the sociopaths to demonstrate their assholery on. Most of the cast I either give no fucks about or actively want dead, and the few I do kinda maybe like I have zero illusions about having any form of non-doomed future, so I feel like getting engaged is actually disincentivized.
I watched it because it was well done, out of a kind of academic appreciation, but I didn't really have fun, and in fact found the whole thing rather a mood killer. The last straw was how basically a whole season went by made of mostly filler with almost nothing actually happening, because the producers apparently decided to split one book into two seasons. To buy time waiting for the next book to come out, maybe? Couldn't have been to do the book justice, cause like I say: mostly filler.
Battlestar Galactica (reboot):
Similar to GoT, but worse, because while GoT is going nowhere, it's at least pretty honest and internally consistent in that. BSG lied. BSG's writers told their audience that they, like the Cylons, had a plan. Halfway through the series it started to become clear this wan't actually the case and never had been, for either the writers or the Cylons.
Plus the high asshole quotient of the cast wasn't as well justified. In GoT it's believable because the culture of that world raises people to be assholes, but in BSG you have a ragtag fleet that's a random cross section of more or less a modern first-world culture. I live in a modern first world culture. There's a lot of assholes, sure, but it isn't 98% asshole. More like 40%, depending on circumstances. There's at least enough non-assholes to keep it slowly lurching forward into a better world, instead of collapsing into... well, Westros.
I stopped watching halfway through season 2. Like with GoT it was hanging by a thread and I was really only watching it out of academic interest at that point. The writing was already starting to annoy me with how transparently the characters would spend a couple eps slowly leveling out into tolerable human beings while the writers were off distracted with character assassinating someone else, only to have the writers suddenly notice that "hey! X hasn't been meeting their 'conflicted' quota! Quick: make 'em do something shitty!". It was like all the characters were on leashes, always being yanked back to the arbitrary minimum mandated level of dysfunction, then wandering back out in the direction of growth only to be yanked back again. And again, and again, and again, in a steady, predictable rhythm.
Again: GoT has shittyness built into the culture of Westros, so the characters being shits is organically self maintaining. With BSG the writers' hand could be seen just reaching in and arbitrarily making people do stupid crap whenever a character was in danger of becoming no longer sufficiently dysfunctional.
The last straw was when Pegasus the technicolor war crime wagon showed up and everyone limply wrung their hands a little and then decided they were totes cool with all the assorted shithorror of it's crew once Cain was under a bus. At that point I threw up my hands and was like: "Y'all deserve each other, and I don't care if you live or die anymore".