When Wash died, I was pissed. I thought, "That's it; it's over. There will be no more FireFly because Joss is going to kill every single one of them."
And that was the point. We were supposed to be scared about the characters, because that's how dangerous the situation was. Joss took FireFly out of the safe TV zone where all the plotlines are tidily knitted up with the picket fence ending and into the realm of movies where characters can die and the crew may not be around to fly the ship for one more adventure.
And it reminded me that Zoe is one tough bitch. She survived a war and the Battle of Serenity Valley where she watched her camrades and compatriots be butchered by the Alliance. She watched her love die and knew his body would be eaten and desecrated by the reavers behind them. But she's a survivor, and she kept fighting, perhaps with more recklessness than usual, but with every ounce of believable vengence.
If Joss had left FireFly in fluffy, TV candy-land, then the movie would have sucked and there would be no hope of subsequent movies. Some people believed that the tv series might one day be revived, but without convincing new fans to love FireFly by selling them with a hardhitting movie, there won't be any more movies, much less a new tv series. No, I don't think that he sold out his fans. I think he gave them the real, gritty, true-to-the-'verse story in which their favorite characters actually risk their lives.
Yes, it was a shame that Shepherd died before we could learn more about him, and some of the other characters were glossed over. But by giving us this hard-hitting movie, Joss gives us the best possible chance of a future for our beloved FireFly. And there's nothing stopping him from making one of those subsequent movies into a prequel to this movie.
I've heard from many people that Joss is always willing to sacrifice a character to guarantee the future of a show. That doesn't mean he kills characters at a whim, but that they are servants to the story. Although I have never cared for Buffy or Angel, it is for this dedication to story and character (i.e. being true to the characters), that I adore Joss Whedon. At least with Wash, Joss gave him the flight of his life and saved the crew from both the Alliance and the fleet of Reavers before he expired.
And honestly, the response I've heard most from people who hadn't seen FireFly at all is a genuine and burning interest to go back and watch the series. And that's what the future of FireFly depends on.