I'm always intrigued by attempts to categorize television series, such as David Auerbach's American Reader essay, "The Cosmology of Serialized Television," that accompanies this chart:
Read the whole thing to get the idea, but Auerbach divides TV dramas into the categories of steady-state, which includes almost all long-run shows before Hill Street Blues ("focused on the short term, with little attention paid to creating suspense toward some distant plot resolution that would signal, if not the end of the show, a major turning point"); expansionary (mythology-heavy shows that "simply [enlarge], with new plot elements and characters injected into it"); and big crunch ("attempts to merge the focus of the Steady-State model with the long-term suspense of the Expansionary model, putting in clues that will mostly, hopefully, add up in the end").
But like most typologies of art forms, the chart above is neater than reality — that is, some shows slosh around in different boxes. Auerbach is quite harsh on Mad Men for its ramblingness ("the medium has forced [series creator Matt Weiner] to promise what he cannot deliver: a coherent story"), but I think most of the show's fans see it as a collection of short stories, not an epic novel that has to end with some kind of Big Statement. Like most sitcoms (of which I'm a big defender), the enjoyment of Mad Men is in seeing deeply etched characters respond, as we do in real life, to situations that force them to make moral decisions.
Similarly, Auerbach's dismissal of the two-character "Fly" episode of Breaking Bad ("beautifully shot but structurally pointless") suggests that he sees each episode of a serialized TV series as simply part of a whole, rather than as having any value as a stand-alone work. I would argue that the mark of a good TV series is that each episode makes sense on its own, not only as a piece of a puzzle — which explains why I'm not a huge fan of sci-fi or fantasy shows.
For a rebuttal of Auerbach's piece, read the always-insightful Jamie Weinman ("TV Shows Don't End Satisfactorily — So What?"), who points out that endings are rarely satisfying in any medium:
Endings are hard because there are very few of them available; you know the old saw about how death and marriage are really the only things that satisfy anybody — and even death doesn’t really count in any fantasy-based story. [...] So that means judging a work by how well it wraps up can be a bad idea — or at least it’s an idea that privileges coherence over everything.