It seems like most long running shows that have a hard time ending and tend to suck. Beloved shows like Seinfeld, How I met your Mother, Lost and others just have terrible endings. Why can’t they finish as strong as the show? For the record I loved the ending of Hart of Dixie.
In the case of How I met your Mother,the creators had an ending in mind and stuck with it, when the direction the show had taken tended to require a modified ending.
You know, I was not a big fan of the Seinfeld series finale at the time, but when I see it in reruns, I think it is as funny as anything else they did in the last couple seasons. Maybe it’s just that people’s expectations get so ramped up that it’s hard for the actual finale to meet those expectations.
In some cases, it’s because the finale for a 30-minute show is a bloated 90-minute (or more) mess.
It was a clip show.
I thought Vikings and Downton Abbey were well done. Now, if Once Upon A Time can pay attention to the writing on the wall and take a graceful bow…
ST:TNG and **DS9 **had good endings too.
The Seinfeld finale was not a clip show, although the episode preceding it was. In the finale, they were taking a private jet to London but had to make an emergency landing in Massachusetts, where they got into legal trouble, basically for the same anti-social behavior they’d engaged in all along.

You know, I was not a big fan of the Seinfeld series finale at the time, but when I see it in reruns, I think it is as funny as anything else they did in the last couple seasons. Maybe it’s just that people’s expectations get so ramped up that it’s hard for the actual finale to meet those expectations.
I think it’s one part this and one part: If you’ve been told you’re being laid off in two weeks, how much of an effort are you going to put in at your job?

It was a clip show.
There was a half hour clip show at the beginning and then the 90 minute episode where Jerry and George sell their show, take a trip to France only to have their plane crash in Massachusetts(?) and for them to be arrested and tried for violating the good Samaritan law. Very little of that was a clip show, though they did have a reunion of many of the eccentric minor characters who appeared on early episodes.
I liked it, though there was no way it could have lived up to expectations.
ETA: Missed Dewey Finn’s post, so, uh, what he said.

I thought Vikings and Downton Abbey were well done. Now, if Once Upon A Time can pay attention to the writing on the wall and take a graceful bow…
ST:TNG and **DS9 **had good endings too.
Vikings isn’t over:
http://www.denofgeek.com/us/tv/vikings/253865/vikings-season-5-release-date-and-other-news

I thought Vikings and Downton Abbey were well done. Now, if Once Upon A Time can pay attention to the writing on the wall and take a graceful bow…
ST:TNG and **DS9 **had good endings too.
Vikings is still running it’s just between seasons.

I think it’s one part this and one part: If you’ve been told you’re being laid off in two weeks, how much of an effort are you going to put in at your job?
How many series that manage to have a finale get canceled with only a two-week notice? Usually, those are long-running series where the producers and cast know going in that it will be the final season, such as Seinfeld or Everybody Loves Raymond.

Vikings isn’t over:
http://www.denofgeek.com/us/tv/vikings/253865/vikings-season-5-release-date-and-other-news
Dammit, I meant Black Sails. :o
I think the factors that make a good TV series work against having a good ending. In the Seinfeld case, there really wasn’t a plot so there couldn’t be an ending. All that could happen was that the show ended, which is what happened.
For plot heavy shows, it becomes incredibly hard to maintain the series over several years (e.g., Lost). Plus there is the expectation of wrapping everything up. Good luck with that. In other words, something that could be wrapped up should have been wrapped up ages ago, and plots that can span several years probably can’t be wrapped up.
Monster of the Week shows are probably the best suited for a good series finale since the season can build up to defeating a big-bad. I bailed on Buffy before the end so I don’t know if that was a good finale or not, but I can see the potential there for something good.
Star Trek is a sort of Monster (or Mystery) of the week show. And TNG had a good finale.
In a sense, the a good finale stands on its own as essentially a good movie of the week set in the series universe. Once it takes on the bloat of X number of years of history it is doomed.
I think series finales usually suck because they violate the very premise of the show. The show was conceived as an on-going story, not something with an ending. So when they try to put a bow around it, it feels wrong somehow (at best) or fails miserably (at worst).
My favorite “finale” episodes are the ones where nothing really changes. You get to believe the characters are still going on with their lives the same as before, they just aren’t bothering to film it anymore.
That said, some of my favorite finales where they actually did “end” things were: Magnum P.I., Burn Notice, and of course Newhart.

How many series that manage to have a finale get canceled with only a two-week notice? Usually, those are long-running series where the producers and cast know going in that it will be the final season, such as Seinfeld or Everybody Loves Raymond.
The length of time is irrelevant. Most people lose the wind out of their sails once they see the finish line approaching.
So yes, even if I know a year in advance, sure, I’ll do a good job for 11 months or so, but during those last days… “Meh”.
On the flip side, I think Angel did a fantastic job with its two-part finale. They had roughly six weeks (if memory serves) to write and film the finale, because everyone involved with the show fully expected it to be renewed for a sixth season. When it wasn’t – and that announcement came in mid-February of the final season – Joss Whedon and the team scrambled to bring some closure to the show.
The fact that they did it – and did it brilliantly, in my opinion – in such a short timeframe remains one of the most creative efforts I’ve seen from a television show.

How many series that manage to have a finale get canceled with only a two-week notice?
Pushing Daisies. But they shot one final scene to give it some closure.
Awake was the same – it closed things nicely (but still left room for another season, if one had happened).
Basically, some are good and some are bad. Newhart was brilliant. Life on Mars (US version) was awful.
The Dick van Dyke Show also had a great finale, where they did some things they hadn’t tried during the run of the show.
Finales tend to be written more as “a thank you note to the show’s fans” than as a normal episode that just happens to tie up loose ends.
Also, in a lot of cases, something happens on the finale that kills the show’s dynamic - for example, the last episode of Barney Miller has the precinct being shut down, and The Mary Tyler Moore Show ended with everybody except Ted being fired… However, there does tend to be a little “wiggle room” written into more recent finales, just in case they want to have a reunion episode; I think the writers of Bones mentioned that they did this. Besides, you could always just retcon the finale, the way The Bob Newhart Show did (in the end, they move to Oregon as Emily got a job there, but in the reunion show - the one after Bob’s dream that he owned an inn in Vermont - pretends this never happened).
In the past, series finales tended to be better, IMHO. The ending to MASH* was quite good. The ending to St. Elsewhere was excellent. I loved the ending to Magnum, P. I. (the real ending, not the one they did the year prior, though that wasn’t too bad, either).
But after MASH*'s ending, the whole concept started getting a bit overdone, like they have to jump the shark in the process. The ending to Moonlighting was an example. After a while, they just started getting ridiculous, trying for too much. That’s especially true if the series gets cancelled before it anticipated being done.