I don't want to write yet another essay about The Death of the Sitcom (the sitcom has been dying every few years ever since TV began). Instead I want to write about how sitcoms got to be the way they are today, starting with one little structural element: the B story.
I think it's fair to say that we now expect sitcom episodes to have a subplot, a "B" story -- a small story involving characters who are not heavily involved in the main plot. Some shows, like Seinfeld, will have the main plot intersect with the subplots at the end; others keep them entirely separate. But one way or another, B stories are so familiar in sitcoms that a primer on how to write sitcoms casually mentions that "Each show has more than one story going on."
The thing is, the B story seems to be a relatively recent development in sitcoms. At least, when you look at sitcom episodes from the '50s or the '60s or even the '70s, you almost always find that they tell one story, no other plots, no detours, no nothing. Go through the four DVD sets of The Dick Van Dyke Show and you won't find a single subplot; same with I Love Lucy or The Honeymooners or All in the Family or Three's Company. The traditional sitcom structure seems to be: set up a story and stick with it. Regular characters who are not involved in the main story will get a token line or two, but not an unrelated story of their own.
What I find interesting about this is that sitcoms back then actually had several minutes more running time than current shows, which means that as running time has shrunk, sitcoms are including more stories per episode (which means even less time to tell the main story). I guess part of the explanation for this is that sitcoms have gotten faster-paced, with shorter scenes; as sitcoms are less inclined to spend a lot of time on any individual scene, they become less inclined to spend a lot of time with a particular story.
Anyway, where did the B story start? I don't think I've ever seen a true B story in a sitcom from the '50s or '60s. Some of the '70s sitcoms from MTM, like The Mary Tyler Moore Show and The Bob Newhart Show, would sometimes include what is now referred to as a "runner," a little running gag that pays off at the end. There was an episode of The Bob Newhart Show where the main plot was about Mr. Carlin being named in a paternity suit (by Loni Anderson, no less), but which included a runner about Bob getting a telephone paging service that doesn't work. This wasn't really a B story, though; it was more a piece of comic business that was strung through the show -- a way of giving a certain structure, and a payoff, to the little physical or verbal jokes that characters do in between plot points. (You know what I'm talking about: Buddy and Sally are joking around about something or other at the beginning of the scene, until Rob comes in and mentions what his problem is this week. The "runner" is a way of taking what would otherwise be filler bits and making them add up to something.)
M*A*S*H may have started the trend toward multi-story episodes; certainly by the later seasons there was almost always a B story. But I don't recall the early seasons of M*A*S*H having many true B stories; they would sometimes do a multi-story episode as a gimmick ("Dear Dad"), but most of the early episodes tended to pick a story and stick with it, much like Hogan's Heroes (where much of the staff came from). So I think the first sitcom to use true A and B stories on a regular basis was Barney Miller. Initially, the use of multiple stories was supposed to be a way of connecting the home and work lives of Barney; the idea was that it would be divided equally between his problems with police work and his problems with his family. The producers and the network soon realized that only the police stories were interesting, and the plots with Barney's family were soon dumped. But the multi-story format remained; every episode had a main plot but at least one other, and sometimes several; an episode would cut between these plots, sometimes connecting them but more often not. It gave the show a feeling of verisimilitude you didn't get with previous ensemble sitcoms: no, people at work (particularly police work) aren't focused on only one problem at a time; different things are going on at the same time.
After Barney Miller, more shows started incorporating subplots, but not a lot. The B story tended to be more common on shows like Barney or M*A*S*H that had dramatic as well as comic elements, and that might benefit from leavening a serious A story with a comic B story. But the Paramount shows that dominated the late '70s -- Taxi, Mork and Mindy, Laverne and Shirley -- basically stuck to the one-story format. Even Cheers, in its early years, didn't do multi-story episodes very often; the supporting characters tended to sit around commenting on the main story, not doing stories of their own.
The Cosby Show, which revived the sitcom the last time it was pronounced dead, probably had a lot to do with establishing the B story as an important part of a sitcom episode. Cosby episodes were deliberately plotless, so the main story often wouldn't fill the entire 24 minutes; and like Barney Miller, the use of multiple stories lent a certain feeling of realism, of parents dealing with all their children's problems at the same time. Anyway, after the success of Cosby, more and more sitcoms started to include B stories on a regular basis. And by the early '90s, B stories were so common that a show like Seinfeld could play around with the multi-story structure by tying all the stories together at the end.
There are a few sitcoms now that almost never use B stories; Everybody Loves Raymond is one such, and that's one of the reasons it seems like a throwback -- instead of a fast-paced, multi-story show, it's a relatively slow-paced show where they set up a single problem and then stick to it, with all the supporting characters focused on thinking and talking about that particular problem. I'm surprised that there haven't been more "throwback" shows, particularly since running time is getting so short.
Anyway, I have a feeling I've missed some milestones in the main story of the "B" story, but that's a (relatively) short primer.