The thing about good stories is that they end. A series can be popular even if it has no particular end, but it is almost never considered classic. As an example, things like Pokemon and CSI don't actually end. They merely continue on in infinite permutation.
I think this is largely because stories are mostly about changing your viewpoint. That is to say, a story starts with the audience seeing one thing, and it ends with the audience seeing quite another. A series can have dozens of "ends" - one or two for each episode - but it only has one final resting place for the viewpoint: the last episode crystalizes the shift.
I don't know if I'm being clear, but what I'm trying to say is this: over the course of a story, the audience is drawn into seeing things the story's way. This emotional and intellectual involvement in the characters, the situations, and the world itself lends great power to the story. An ending takes all that momentum and slams it into a final "view" of the world. Generally, ending with the good guys winning.
Even if an ending is poor, the last episode of a series you've watched is very potent. You build up an intellectual and emotional involvement in the world just by watching - even if you're not particularly enjoying it. For things such as movies, with their shorter time spans, you need to have extremely efficient draws to get the same level of involvement you can get from a series. It's just the fact that the series simply has more time to let the audience grow to like the world.
I want to talk about endings. I had to go talk about everything else, first, because I wanted to explain why endings are so critical.
I want to explain that it really isn't hard to have a good ending. In fact, it's not even hard to have a good ending that doesn't end your series. The problem is, your ending is only as good as the view it leaves the audience with, and that view has to be something spectacular in comparison to the normal view.
Some series who don't want to alter their "winning formula" try to achieve this with great fights, large explosions, and walk-ons dying. There is some catharsis to be found in that sort of thing, I suppose, but an ending is called an ending because the story is over. The story has accomplished what it set out to accomplish. It has shown the audience whatever it had to show. No worthwhile story has an unlimited number of episodes in it.
The ending is the most powerful part of the whole story, because it has the weight of the whole story behind it. You've probably noticed this. If you've ever watched four or five hours of a series all in a row, finishing up the last episode, you'll notice that the ending leaves you feeling very different for several hours - even if you were displeased or unsatisfied with it. The power of an ending simply cannot be overstated.
The problem is that much of the power of an ending comes from the brutal finality of the ending. Once a story has ended, it cannot be ressurected or continued without diminishing the impact of the ending. You put in a series that you like, you watch it, it ends with a potent and beautiful set of scenes. You are in awe for all of two minutes while you switch the disk out for the next plot arc. Then that beautiful final viewpoint is crushed under the heel of the new plot arc's viewpoint.
But fully utilizing the power of an ending is, perhaps, even worse. Because that means that your wonderful journey is over. Forever.
After all, how many people have burned to watch more of a series that just ended well? I know people who would happily watch another eighty seasons of Babylon Five or Rah Xephon. The endings were permanent in a way which was almost brutal to the audience.
In my opinion, endings do not need to be permanent. They do, however, need to be temporary. Meaning that an ending viewpoint has to be dwelled on for a fairly significant period of time before being replaced by the next story arc's preliminary viewpoint.
The way you do this is simple: encourage the audience to watch extras after the ending. Whether it's the ending of a plot arc or the final ending of the show, extras serve two purposes:
1) They give your story a feeling of being made by humans. Moreover, by humans the audience now knows. This doubles or more their chances of evangelizing and buying other products you've put out.
2) So long as they aren't radically inappropriate, they give the audience time to soak in the final viewpoint. Extras don't shift the audience's viewpoint like the next story would, because they are not intended to be stories.
For the sake of all that anyone has ever held sacred, put extras on that final disk, whether it's truly the last disk or simply the last disk of a plot arc. It doesn't matter what these extras are. One of your actors jabbering on about how he kept trying to date one of the actresses is fine. Production sketches. Fan art. Anything.
To be honest, I think that out-takes are probably the absolute best extra you can put on a disk, so long as your ending was reasonably upbeat. Any scriptwriter worth anything can write you some out-takes if you don't have anything suitable. Also, light personal humor about your various actors and difficulties is also fantastic.
If you are planning on releasing a story without extras, you are shooting yourself in the foot. The twenty-first century marks the beginning of the Extras Age. The audience wants to hear about all your stupid little troubles, all the ideas that didn't make it in, all the lame jokes your cast makes. And you want them to hear this stuff, because it doubles or triples the likelyhood that they will evangelize and buy whatever else you've put out in addition to radically enhancing the potency of your ending.
Extras, folks. Seriously.