(In which I natter on about television, because I’m working on an editing gig and don’t want to distract myself by talking about writing.)
I think it’s fair to say that some TV series end better than others. Old-fashioned push-the-reset-button drama series and sitcoms didn’t have to do much work to tie off the story — M*A*S*H had the end of the war, The Mary Tyler Moore Show had the tv station close down, other shows just moved without a ripple into the eternal reruns of syndication — but the development of arc-based storytelling put a new burden on television writers, the need for giving the faithful viewers a satisfactory denouement, and I think we’re still seeing them figure out how to do it.
The X-Files wasn’t necessarily the first of the arc-based shows, but it was definitely the one that showed everybody else How Not to Do It. Its sins were many — lack of a clear backstory, failure to end at any one of several perfectly good stopping places, failure to redeem plot coupons the audience had been holding onto for several years in some cases — but it could, I think, have mitigated at least some small portion of its general Fail if it had only done one thing.
That thing? Establish some kind of victory condition early on, and see to it that at least some of the show’s sympathetic characters survived and met that condition. The X-Files disappointed us on almost every count: Mulder never found Samantha or got justice for her abduction/death/whatever; the big conspiracy was never revealed or thwarted or destroyed; Mulder and Scully never achieved vindication and professional recognition (in fact, their FBI careers basically go down the toilet); Scully never got to have and keep a kid; hell, not even the damned aliens got a satisfactory resolution, since they never got to do their Big Colonization Thing while we were watching, either.
Buffy the Vampire Slayer, on the other hand, while it didn’t please all its viewers on all counts, succeeded in that one big thing. The show gave us, from the beginning, a main character with a big problem — to wit, her mystical destiny as “the one girl in all the world” — and at the end of the series not only have she and her friends defeated the final season’s Big Bad, she and they have succeeded in rewriting the terms of her destiny so that she is no longer forced to carry that burden alone.