About Spinal Tap, if there had been a story, a making of an album or something, I could have appreciated it, but as it was it there was just no narrative structure. No matter how good the jokes are, I can't enjoy something that is barely held together for well over an hour. It could have been a good 15 minute sketch, but no more.
Many people love Firefly and others speak of potential. To me, there was just the the chemistry and the hints of stories behind everything. I just want to know what happens between Mal and Inara, how Rivers' powers evolve, and who the hell Shepherd Book actually is. I want to see more of Jubal Early. I want to see the relationship between the Doctor and Kaylee evolve. This is what makes it so great. There is enormous potential for further greatness, but the series also gets you so involved on so few episodes that it deserves all the praise in the world. I can't say there's many series where I like and care for every castmember after just a couple of episodes. Maybe it would have gone downhill from there, who knows, but it's the promise that keeps it so alive in many peoples' eyes.
In retrospect, I think Serenity is a great film on its own, but it does not manage to connect to the atmosphere and feeling of Firefly - neither does it give a resolution to the storylines, which in the series was much more about the characters than the story of the Reavers and the universe.