Personally, I believe that while the show was not as godlike as the diehard Whedon fans believe, it also was a world apart from Whedon's previous work.
In other words, Firefly was where Whedon was /beginning/ to get serious and was consciously struggling to raise himself out of the "hack" category, putting the good parts of his abilities to use. People who dismiss the show as being "just like all his other stuff" are reading in what isn't there; not all the characters are his stock characters, they actually have different voices and personalities, and the universe he was setting up had much more nuance and was damned intriguing.
Truthfully, Firefly showed the most potential of any major sci-fi to make it on television in years, especially because it showed signs of being a postmodern space opera: its rethinking of space opera tropes was more in line with modern authors like Alastair Reynolds than with the Star Trek-age ideas that have fueled (and been mined clean) mass market sci-fi for decades. It was a better show than Moore's BSG in many ways. (For one thing, while it was clear Whedon and Mineer were interested in turning sci-fi on its ear, they didn't show open disdain for the genre that Moore did and caused BSG's mythology to fall apart.)
So yeah, there's a really bitter pill to swallow. To draw a comparison: what if say, NuBSG had been suddenly canceled after the first 13 episodes, at the episode 13 cliffhanger? How much brutal sting of wasted potential would have been felt, and how much bad blood would have been directed at the network involved? I suspect it would have made the Farscape cancellation look like a girl scout campfire.
While lots of "browncoat" fans have been very annoying, I think you can argue Firefly getting screwed over is maybe the biggest missed opportunity in television sci-fi in the 21st century.