I sort of disagree on two levels. One, I think that's true of almost any writer, it's just more noticeable with writers who write primarily in the same genre and write in a stylized manner, as Joss does (but hey, Nolan does it too... his femmes fatales are all very similar, as are his brooding antihero protagonists tortured by the horrible death of a loved one, usually a woman).
And two, I think you can definitely see similar character types across Whedon's work (Xander/Topher/Wash, for instance), and those characters tend to talk similarly, but within each thing he creates, I feel he's very good at differentiating character voice, which is one of the hardest things to do as a writer (i.e. Wash may sound like Xander at times, but he doesn't sound like any of the other characters in Firefly).
He does write stories where all the characters are witty, it's true, but they are witty in their own way in a way that's true to who they are as people. IMO anyway.