I think most writers fall into the trap of creating similar voice for all their characters. Stephen King's characters are that way. David Eddings's characters were too. Anne Rice's, Terry Pratchett, the list goes on. Hell, I've noticed it in my own writing, and it bugs the shit out of me. I constantly struggle to give each character a voice of their own, while still remaining true to my style and voice. It's because the author is so much a part of each character, you can't help but have some of your own personality and voice creep into theirs. Whedon does as well as any in that aspect, and believe me, it ain't easy!
And to those saying that bringing back the show would be a tremendous letdown because all the momentum went out of the series at the end, I'd say that as a series with some of the most loudly voiced calls for a return, it'd do just fine. They'd stir up a huge frenzy of excitement over a reboot, and as long as they picked right back up where they left off, kept the plot moving as it was in the series, and continued Whedon's original vision for it, they'd be just fine. There wouldn't be a letdown as long as they stayed the same course the original series was on, and the only way for that to happen is by setting the movie aside.