Interesting question (well, I hope it is...) At what point does a series' mythos become an impenetrable barrier to attracting new viewers?
Obviously, I ask because of a certain television series that has just recently reached its conclusion. I'm pretty sure that it's a bit late now for me to start watching 'Lost', seeing as how it's not actually airing anymore; but even before last night's series finale, I was already getting the feeling that if I wanted to watch the show, I'd need to do it by buying DVDs and starting from Season One. There are plenty of other shows like that, as well: The X-Files, Babylon 5, even the Stargate serieses tend to get pretty deeply into their backstories by a few years in. (Doctor Who, a show that's in it for the long haul, tends to resolve everything and start over every five years or so, but it's the exception to the rule.)
So how long does it take for a show to become something that "you really have to start at the beginning" to watch? Is it a good thing, or a bad thing? And when did it start?
Taking the latter first; as a science-fiction trend, I'd say the push for longer and more involved story-arcs arrived alongside the home video market. Before that, we had soap operas, which obviously involved a lot of involved backstory...but we'll get back to that later...but really, the episodes of most sci-fi shows before Babylon 5 could be seen in any order without confusing the viewer too much. Occasionally, you'd find a new doctor on ST: TNG (or a new Doctor on Doctor Who) but for the most part, producers didn't think they could do stories with long, involved arcs, because they couldn't guarantee that people would see the episodes that set up the episode they were watching right now.
Home video changed all that. Even before DVD, series like B5 and Highlander were putting out big, chunky season sets that took up whole bookshelves. Sure in the knowledge that people would be able to follow their plotlines for only a minimal $100 investment, they could do longer, more involved storylines. DVD only accelerated the evolution of the trend.
But unsurprisingly, some people didn't necessarily want to shell out $50 (it got cheaper by the time DVD rolled around) to catch up on the show everyone was talking about. Or, for that matter, to invest thirty-two hours of viewing time, either. For those people, comments by the fans along the lines of "oh, you really need to watch it from the beginning to get the full effect" might as well have been saying, "Too late, buddy!"
Which is probably not what the producers wanted. Because let's face it, for all that shows like "Lost" have more involved backstories, it's still not like you're watching one long movie cut up into a couple hundred hour-long chunks. Fans tend to overestimate the impenetrability of their favorite shows and continuities, perhaps because it makes them feel a little more special to know that they're one of the Chosen Few who can understand everything that's going on in their favorite show. They love to start telling people that a show really rewards patient viewers as soon as the second episode, creating an almost self-fulfilling prophecy of confusion. But soap operas specialize in tangled plots that go on for years, and they've been able to consistently attract new viewers. Obviously, if what's going on that moment on screen is interesting, people will be willing to sit down and watch until they've picked up all the stuff they didn't understand.
Unless the series is, y'know, over. Then it probably is a bit too late.