I've long struggled to be clear about what voice is. I've fumbled around on panels and often times resorted to the true-but-not-helpful "I know it when I see it."
Jessica Faust at BookEndsLLC nails it here.
Voice is one of the ways I can tell where a book goes on the shelf without being told the category.
Posts like that are one of the reasons the BookEnds blog is on my must-read blog list. It should be on yours too.