Xena, The fourth season. The show went from light hearted adventure to semi-serious and mystical. The clashing mythologies, a Jesus analog, time travel, the death of the Olympian gods, Gabrielle with short hair! The departure from the Greek-Roman myths to encompass far eastern styles I thought was a mistake.
The series ended on a downbeat note and a I beleive a betrayal to the fans.
Does Red Dwarf count? when chris barrie left as Rimmer and kochanski joined the crew it definitely lacked the humor that was generated between Rimmer and Lister. They never really got it back, even when Rimmer kind of returned in the last season.
So she does all those Lifetime Movies as some sort of revenge?
/hack-joke
Well, at least in contrast to the episodes of Star Trek, what made this episode unique, in a way, was that there was no such thing as modern medicine. Basically, the “germ theory” of disease never took off. Maybe Louis Pasteur died as a kid?
I also really like when, for over half the episode, we just assume it’s the worst disease ever and can’t be treated with antibiotics, and then the alternate-universe Quinn just goes,
“What’s penicillin?” and Arturo finally realizes why they can’t cure the disease.
Personally, I just hated the whole “cro-mag” sub-plot that dominated season 3. Season 2 was alright, the prof. died, but at least Kari Wuhrer was hot. 
I came in here to post exactly that! Good call.
What about Meathead’s entrance into “All in the Family?”
ETA: I guess he was always in there after all and what I’m recalling is a flashback scene.
Never mind…
Ellen, the post coming out episodes. It started as a funny, light comedy and morphed into I’M GAY AND MY LIFE NOW REVOLVES AROUND THAT! It really stopped being funny at that point. While I certainly applaud her doing the coming out bit, the show really lost it’s way after that.
Does losing a writer count for this thread? I’m just going on a probably faulty memory here, but I seem to remember reading that Gene Roddenberry was dissuaded from writing scripts after a lousy first year, and the series improved after better writers took over. This was right about the time Riker grew the beard, IIRC.
Supposedly, during the taping of the first-season episode where Alex lost his virginity, Family Ties creator Gary David Goldberg listened to the audience’s laughter and said to Baxter, “If you want out of your contract now, I wouldn’t blame you.”
It’s hard to remember now, but Newhart was extensively retooled after its first season. His new foils (Michael and Stephanie and Larry, Daryl and Daryl) seemed to come from parallel universes and makng Dick Loudon host of “Vermont Today” allowed Newhart his classic shtick of sitting there, being bemused. In fact, the whole show was something like Green Acres Moves to Vermont.
The X-Files almost shifted into a comedy for large parts of season 5.
Good call! I can’t think of why this made a difference, but the show also went from being recorded on tape to being recorded on film in the second season, and the whole look of the show changed for the better. Why that seemed to be so, I have no idea.
As I’ve noted a couple of times before, for it’s first five or six episodes Lost in Space was actually a serious SF show, with an established action/adventure actor, Guy Williams, as it’s star. Then Batman started the late 60s camp craze, Dr. Smith turned from a villainous saboteur into a mincing dandy, and the rest is history.
"OHH THE PAIN THE PAIN… William my boy I just can’t go on…perhaps if you rubbed my back a little "
(ROBOT FLAILS)
“DANGER DANGER WILL ROBINSON!!!”
That shift seemed to also hit when it switched from Black and white to colour.
Star Trek: Voyager After Kate Mulgrew left the show and they killed off Janeway, it returned to it’s original promise.
Oh wait, that never happened. Shit.
In the middle of it’s run Angel fell into a lot of grim-n-gritty stuff that I didn’t like and I stopped watching it. Then in the fifth season, it pulled out of that stuff and returned to being a really good show, finishing up rather well.
Enterprise was so completely ruined by Braga and Berman that not even a worthy fourth season under a different producer could save it.
All in the Family changed enormously after Jean Stapleton left. Archie Bunker basically became a big teddy bear. The show even changed its name, but I can’t remember to what.
Trading Spaces changed significantly, and not for the better, when Paige Davis left. I’m glad she’s back.
In Coach, Craig T. Nelson was a mean, curmudgeonly, but funny character. But as the show wore on, it seemed like every show became a PSA. Becker fared a bit better, but same deal.
It is interesting that you see it that way. To me, pre-coming out, Ellen was a way too light comedy, occasionally worth a chuckle, but mostly aimless. Afterwards, it was about something, and to me at least, more consistently funny.
Not a TV series, but a good example is Hannibal Lechter. In the first two books, he was this horrible scary guy that was just creepy enough.
In the last two, he became a routine slasher. Ho hum.
Different strokes. 
Though I can honestly say you’re the first person I’ve ever heard express that opinion. (Not saying your opinion is bad, just that it’s new to me)
I never saw it that way. The coming out episode is one of the all-time funniest things on TV ever. But every episode after just got worse and worse until the show was finally canceled.
Veronica’s Closet. OK, I’m not proud that I watched enough episodes to know this, but at the beginning of the run, Veronica was a quirky but basically capable entrepreneur keeping her own business afloat. By the time the series was canceled, she had become a useless figurehead of a company run by a man. When Rebecca Howe on Cheers had de-evolved I blamed the writers, but after seeing a second Kirstie Alley character do essentially the same thing I began to suspect she was somehow at fault.
It’s been years since I’ve seen it, but I remember Night Court starting out as a goofy workplace sitcom with a zany cast and succumbing to chronic “very special episode” syndrome later in its run. Maybe having a couple of actors die will do that to a series.
This might not be a popular opinion, but Arrested Development took a severe turn for the worse in its aborted third season. In the beginning, as flawed as the characters were, they (well, some of them) retained enough humanity that I still kind of cared, a little, what happened to them. Only the parents were truly monstrous. The others – well, my favorite moment from the whole series was when Michael’s mother tries to frame him for running down GOB and GOB, says something like, “But you don’t hate me. In fact, you kind of like me.” Michael, somewhat surprised, says something like, “You’re right. I do kind of like you.” And yes I know the third season explicitly mocked the idea that characters need to be at all likable, but that doesn’t mean they were right. (OK, so why am I such a fan of Fawlty Towers? Possibly because it only lasted for 12 episodes, but also because the protagonist was presented as a mean-spirited bastard from the very beginning.)