The thing about Firefly, I feel though, was it was Whedon at his best. He had an amazing cast that in every episode didn't fail (in my eyes) to capture their characters and bring the show/verse to life. Whedon's humor was superb. I look at it like this - he had Buffy and Angel to practice his craft and Firefly was his best because he knew how to run a show and work his humor and excellent sense of a story/world into the script. I don't idolize the guy, but I do see a good writer and show creator. He is underestimated because of his style/humor, which is unfortunate.
The thing about Heroes, as an aside comment, is the writing got worse and worse in each season. I loved the first season. Second could have been better if not for writers strike, and the third just lost it entirely. I couldn't even finish it. The fans died away because of the strange story-lines not the show itself. Whedon could have pulled off several good seasons before the show might have gotten bad. It was much simpler than Angel and Buffy, having a lot less room to get into the strange stuff he did (a lot of which he realizes now were terrible).
I personally think the biggest reason Firefly was so successful (with the fans anyway) was because of the characters. They were believable and drew you in. I liked every character (which is rare for me). They all were great in some way. Mal was about as kick-ass as it got then had his imperfect moments which were usually hilarious. Jane played his slightly dumb hack'n'slash type great, adding a few elements not seen much. And so forth.