Happy Days when they had The Fonz water ski jump over a shark.

Yep, I'm old, and yes that is where the term "Jump the shark" is from.

Finally something I'm qualified to answer!

Family Guy - While more "Flanderization" when all the characters became parodies of their previous selves, the real thing is when all of a sudden every joke needed to be "explained". i.e. someone trips while running and then one of the character says "AHHH HE TRIPPED AND HE FELL". Ruined every joke for me, was done with it after that. Kind of one decision, I'm sure someone said "people don't get the jokes" and ruined the show for me.

House - When they suddenly swapped the cast out. Some people really liked it, for me that was the end of the original premise of the show.

How I Met Your Mother - Lily and Marshall's baby. Having a baby in a show is always kind of the "Okay the romance story is over and the writers are out of ideas" but it was done so badly. Lily was always an extremely selfish character, and she was insufferable through everything, from making the literal choice to have a child based on some fated chance to shaming Marshall for having the gall to - go to work to support all of them while she did... nothing? Did she even have a job? Constant guilt trips and manipulation, she was the worst character but their relationship was just straight-up toxic when she got pregnant.

That 70's Show - Having Donna stay home from college - 4 years of characterization thrown out the window. She was always a strong independent woman and now she's just apparently throwing away her future? She attends remote classes for a while then it just disappears. Then suddenly Eric has this big revelation later that him marrying her might lead to an unsatisifed future? It was all just very clear that the writers didn't know how to handle her being gone - a very common coming of age issue

X-Files - The move to California. Enough said.

The Witcher showrunners were going to have Geralt and Yennifer have a child. This was also the decision that made Cavill leave the show, which just cemented my choice to not bother with the following season.

Do what you will with the overall plot line and stories you wanna tell, but god that is such an egregious change that fucks up so much about the character's motivations since canonically, Witchers are fucking sterile and can't have kids, but Geralt really wants to be a father. That's one of the reasons he attached himself to Ciri.

Not sure if it counts, but I loved the Lord of the Rings movies. Had high hopes for The Hobbit, but the way they portrayed Thorin completely broke it for me. Didn't bother watching the other two.

Controversial take but Star Trek Voyager.

When I was a kid I loved it, it was my favourite Star Trek. then they introduced Seven of Nine and suddenly it became a borgfest.

I liked it because it was all new alien races, stuff that's NEVER been in Star Trek before...and then they were like "no ones watching, lets bring in the Borg."

Well, no one else mentioned it, so I will.

Altered Carbon. Season one was solid. I completely bought Joel Kinnaman as a strung-out anti-hero.

Anthony Mackie? Not so much. Falcon? Yes, that guy emanates Boy Scout vibes, but he looked ridiculous having a drink at the bar. I didn't buy him as the same character for one second. Completely miscast. Flabbergasted at that one decision, though it had nothing to do with the writers.

Jessica Jones. Season one was fucking magnificent. Season 2+ is.... A second season of that show. My understanding is, that they changed the writing staff of the show.

The 2nd episode of "Man in the High Castle" when you realize they just copied the setting and are going to ignore the key elements (and strengths) of the book.

I gave up on the adaption about 5 min into the second episode.

Anything that's been "adapted" in the last decade or so: Witcher? Dog shit. Halo? Hell no. Wheel of Time? Fuck off. Amazon LoTR? Fucking spare me. Disney ANYTHING?!? I'd rather set myself on fire than validate the raping of all my childhood heroes.

Writers are an extinct species. All we have now are cultural vandals.

Dexter. 1 ep after trinity, I stopped. Decided lithgow had a perfect ending.

I started watching 'The 100.'

I gave up when the folks from orbit decided that they had to massacre the ground dwellers.

This was after a dozen of so other massacres.

the 100 after that moment

Doctor who. Every decision after Capaldi.

Game of Thrones.

I wouldn't say "unwatchable" but one thing I really didn't like about the King of the Hill revival was aging everyone up. Mainly because it made Bobby a relatively normal, not very funny adult which eliminated like half the comedy from the show.

Battlestar Galactica. I didn't even really like the show because of the tone anyways... but in the last episode of the first season they throw the female lead in the bed of the male lead who was a "bad guy" for some reason.

I realized they didn't actually care about plot at that point, and were just going for shock value.

Eureka. The time travel, and that god fucking damn bunker

Not really decisions by the writers, but more losing the good cast members.

Misfits and Being Human UK

I was watching Mr Mercedes and the first season was interesting but after the second show in the second season, I quit. I don’t know what the writers were thinking but that was enough for me.

Two words: Daisy Johnson.

Bob's Burgers around season 5 or 6. The tone changed and it was less learning about some ratchet ass setting and meeting the people in it and more about sticking to the established normalized rules for the setting. Mort kinda vanished.