Well, because they were ending the whole thing with that movie, I don't think it was such a terrible thing to do. Shepard's death was sad but not shocking. Wash's was. It showed the seriousness of the situation. I think it worked.
That is a tough question. On one hand, I love Wash. He is my tied-for-first favorite on the show. But on the other hand, that was a hell of a way to end the series, having the the best pilot in the verse' throw out a great quote and then die. It's really all opinion.
Even though I hated the fact that he got killed, he went out on a high note. Wish they could bring back a new Firefly movie, but it wouldn't be the same without Wash. Plus, its been so long since the show/movie came out, it'll be like that X-Files 2 movie that came out and maybe only die hard BrownCoats will go and see it.