I've tried out nearly a dozen new-to-me TV series recently; I'll post more about them soon, but for now I have a general observation.
In many series, there's an overarching long-term storyline; in such shows, each episode usually has its own plot, but also advances the big-picture storyline to some degree. It's sometimes called the show's “mythology” or “arc”; TV Tropes calls it a “Myth Arc.” I don't find any of those terms very satisfying, so I'll just call it the overall storyline.
What I realized recently is that in most shows that have an overall storyline, that storyline is what I'm most interested in—but I think the individual episode storylines are generally what the people running the show are most interested in. The overall storyline is there to keep viewers watching, with tidbits of information doled out in one or two Arbitrary Information Units per episode, but that information is often treated almost as an afterthought.
The specific instance that led me to this thought is Burn Notice, which Kam and I have now watched the first few episodes of. The overall storyline is a spy story, but the individual episodes are more or less detective-show mysteries. (In this show, the two kinds of stories don't even match in genre.) The protagonist claims to be primarily interested in finding out who issued the burn notice, but he spends almost all of his time solving mysteries and rescuing people in distress. (Though I'm told that later in the series there are episodes that are much more focused on big-picture stuff.)
I think the same kind of thing is probably true of most shows that have this structure, going back to The Fugitive or maybe earlier for all I know. (And including that whole subgenre of shows featuring an itinerant protagonist finding someone in trouble each week and helping them.) For me, the individual-episode stories feel more or less like distractions along the path of the overall storyline; whereas I suspect the writers (and most viewers) of such shows see the individual episodes as the point, with the overall storyline being a carrot to lure the viewers to keep coming back.
Note that I'm mostly not really talking about an open-ended overall storyline like “Will Lorelai find love?” or “Will Vinnie survive as an undercover cop long enough to take down the bad guys?”; I'm mostly talking about overall storylines that pose a specific mystery to be solved, like “What alien menace is the government conspiracy covering up?” or “Who issued the burn notice?” or “Who is the one-armed man?” or “What killed the protagonists' mother?” or “Who is Red John?” I'm not sure I can firmly delineate between open-ended and specific overall storylines (and I'm sure there's a big grey zone between them), but they feel to me like different things.
In some cases, the big-picture storyline isn't for a whole series, just for a season. The beginning of the season sets up what looks like it's going to be a season-long storyline, and so I expect that every episode will be focused on that story—and then it turns out that half the episodes of that season don't even mention the season story. When Buffy season 7 did this, I thought it was just really bad pacing, with forward momentum starting and then lurching to a halt every couple of episodes. But then I saw the same thing in Leverage season 3; it looked like the whole season was going to be dedicated to dealing with the Moreau issue, but there were entire episodes where he wasn't mentioned. I eventually realized that I was misinterpreting. What I was seeing as a season-long storyline, the show's writers were seeing as a backdrop, or as a several-episode storyline shown in intermittent chunks over the course of a season.
Recognizing this disconnect definitely helps me appreciate the shows for what they are. But it nonetheless leaves me a little disappointed; it means that most shows, no matter how promising or intriguing their big-picture storylines are, are unlikely to be what I want them to be.
There are, of course, shows that don't distinguish much between big-picture and individual episodes, shows in which most episodes are important parts of the overall storyline. I tend to like that approach a lot—about half of my favorite shows take something like that approach—but such shows can get so immersed in their storylines that it can become difficult for new viewers to find an entry point.
And there are shows that are somewhere in between; I often like those too. Hill Street Blues tended to have overlapping two- to three-episode storylines. Nikita has plenty of standalone episodes that aren't focused on the season-long storyline, but I suspect that even those episodes would be largely impenetrable to someone unfamiliar with the backstory. And so on.
So I'm not trying to set up a set of firm categories here. I'm just observing that in general, if there's a longer-than-one-episode storyline, I'm likely to be more interested in that than in the one-off standalone stories that (to me) interrupt that storyline, even though I think the writers often see the standalone stories as what the show is really about.