Just a quick note to add to Lyda's comment on narrative voice below. I pretty much agree with everything she says, with one notable exception that she actually doesn't address, the multiple-viewpoint novel.
Multiple viewpoint makes for a real challenge to the idea of narrative voice because you have to make a decision between narration in the individual characters' voices and narration in a neutral voice-of-the-novel voice.
If you choose the former, you have to make sure that the switches between character voices aren't so violent that they destroy the cohesion of the book. This can be worked by making sure that your transitions come either at scenes that bridge the gaps between characters, and by sorting the order of your characters very carefully.
If you go for the latter the trick is to keep it from blanding out the non-dialogue scenes. You can get at this by making very slight tone and language variations from character to character and by giving the book a powerful personal voice of its own.
My own personal choice is to lean toward the latter with dashes of the former and I think I've done a fairly successful job of it when I've tried it, though the other Wyrdsmiths might do a better job of making a fair call. The Urbana had six strong viewpoints that I can think of off hand, and Winter of Discontent had four primary viewpoints and nine lesser ones. It's always a balancing act.
What about you o' wise reader? I've learned quite a number of things already by throwing open the floor. How do you handle the tone of narrative voice in multiple viewpoint stories? Am I missing some vital point here? Saying something the strikes you as dead wrong?