Oz
A great, edgy show devolves into a fucked up soap opera. Lee Tergesen, forever typecast as the spineless homo lawyer, finally gets released after years of extraordinary, heart-breaking shit...only to get thrown back in. The finale is a bizarre attempt at Shakespearian tragedy (literally, for a few characters), that falls flat.
Six Feet Under
Really started to meander after the first year, but became intolerable as Nate refused to deal with his rich people problems, and that stupid kid, and the whole dysfunctional shit just spinning in dull circles episode after episode. All to end with Nate's retarded death.
Space: 1999
Season one explored the origins of Mankind, and was a character study within the Monster Of The Week journeyman formula. Deeply flawed, it was going somewhere... The studio-reworking of season two removed all of Johnny Byrne's influence and turned the show into a glitzy embarrassment that petered out towards the end. Not to mention that 80% of the cast vanished, only to be replaced with hideous cliches.
Sopranos
After the third season, the tired plotlines between Tony and the therapist were stretching things. Edie Falco's affair and the overall melodramatic soap opera dealt a hard blow to the gripping appeal of the early seasons. Tony huffing and puffing around, and HBO's erratic hiatus freakouts really drove the last nails into the coffin. Sopranos became the gangster version of Seinfeld: A show about nothing.
Jericho
The first season really grew on me. The triumphant return for an abbreviated second season was a shitfest. No longer post-apocalyptic (even the casualty punch got pulled -- just a few million dead, when 24 cities got annihilated), it became a weird Dark Angel style people versus the corrupt government routine.
Dark Angel
Never strong in the first season, giving her all of her little friends and changing the whole format for the second season was maddening.
War of the Worlds
The Biblically-themed first season where the child of our heroes from the 50's movie version battled the surviving remnants of the original invaders was terrific. Complete with guest spots from the 50's movie, and copious flashbacks, and a loving regard for the timeline and events set up in the film, the first season had all the right sort of thrills. A lovely ensemble cast formed a secret government organization out to get the aliens who, stranded and without supplies, suffering from the illness that killed them in the movie, take over human bodies and walk among us. The catch being that the bodies rapidly decay and fall apart, so they're always changing form.
In the second season, the deliciously evil and always comically desperate "Triumvirate" were replaced by a second invasion of aliens, arriving in stable human bodies. Half the cast were killed off and we flashed-forward about 10 years to a post apocalypse retardo-fest. The Biblical stuff vanished, but they did introduce some weird Martian god. It was stupid.
Remington Steele
Jericho's disgrace called this show to mind. Catchy, funny, cheaply amusing... Until Pierce Brosnan unsuccessfully tried to break his contract and the show fell apart. We learned Steele's true identity, and he and Laura finally did the deed. The show was brought back after a write-in campaign, but was never the same. Every element of mystery and sexual tension was gone.
Voyager
The beginning of the end for the saccharine Star Trek franchise started off fairly early with Voyager. What should have been a fantastic way to inject new blood into the show (single ship lost without the Starfleet/Federation dues ex machina), turned out to be absolute shit. It even managed to pull the Borg's teeth.
Andromeda
Very much the precursor to Firefly, the first season of Andromeda was loads of fun. Hot chicks, great characters, and a fun story. This was pretty much fucked over by the second season and, then, turned into a complicated mess that's unwatchable by the fourth season. I can't even recall the final season...
V
The spectacular original mini-series still stands the test of time. The less accomplished Final Battle didn't quite know what to do with itself but, thanks to all the cast sticking with it, and a huge budget, turned into pure enjoyment. Where things went to hell was when it came time for the weekly series. The Final Battle, which was pretty damn final, turned out to be, oh, not that big a deal. With the money ripped out from under them, the TV series is set in the "free city" of LA after a Visitor re-invasion that is only partly successful (they can't occupy cold climates). We rarely get details on the proper war that rages between the humans and the Visitors and, instead, focus on a trimmed-down group of freedom fighters from the Final Battle who are, basically, chaperoning the Starchild to her ultimate destiny...whatever that is. (To blow up the Visitor leader's ship, which isn't even her doing.)
Despite Diana surviving the Final Battle, and leading the re-invasion, she seems oblivious to Donovan and Tyler and all the other well-known resistance leaders. So they all can run bars and operate in the open and shit like that without the Visitors coming for them. Call me crazy, but the ringleaders of the plan that defeated the Visitors the first time should have topped the list of people to "disappear" after the re-invasion.
Survivors
Terry Nation's excellent post-apocalypse drama is, in my opinion, the best apocalypse show out there. The first season, though marred by the lead character's quest for her kid, is pure fantasy-apocalypse wankfest. The second season, then without Nation's guidance, became Little Apocalypse on the Prairie. The third season is about as far from making sense as possible and, at least once an episode, you have to pound your head against a desk to dull the pain.
Millennium
Season one is just Lance Henriksen being cool. Season two explores the mystery of the Millennium Group, and really gets deep in there... Lots of fans hated it, but I loved it. We also got one of the ten best season finale's in TV -- the entirety of Patti Smith's "Horses" plays over a plague-apocalypse-drug montage as Lara Means dies horridly and Frank arrives not at all in time.
Season three finds that the apocalypse was somehow averted (we don't find out how or why for half the season), and Frank is suddenly on his own. Though the Millennium Group remains, Frank is now opposed to them (so they end up working a case from two angles and sniping at each other in EVERY GODDAMNED EPISODE).
Meanwhile, Frank's new partner has daddy problems and is being wooed by the Millennium Group, despite Frank's constant warnings and even a few obvious object lessons.
Any others?
