Who knows? Maybe the brevity of its run prevented the flaws from creeping in. Buffy was about twice as long as it should have been, and Dollhouse was given way too much time to fail. Joss Whedon seems to be better at coming up with an idea than executing it in the long term. I read one critical review that pointed out that his best two shows, creatively speaking, were basically "Han Solo: The TV show" and "Batman in LA" (Angel). In other words, running off of a sucessful formula. The longer the show runs, the greater the chance of the writers pulling out the lazy sloppy tropes that appear in every sci-fi TV show (grim future; alternate reality; evil twin episode). A lot of what made Firefly work, by Whedon's own admission, was the actors and the way their characters interacted and played off each other. How long before that mixture goes stale or gets ruined by the departure of an actor or Whedon's belief that nothing is real without characters dying (a belief widely shared by lots of genre fans, so there would have been criticism if nine people kept doing such dangerous things for years with no fatalities)?