Consistency: A good voice is consistent throughout a novel. It may get darker or lighter or funnier or sadder, but it doesn’t suddenly shift wildly from whimsical to murderous or psychotic. A good voice is never lost when the plot shifts.
Moderation: Even the strongest voices don’t over-do it. Voices are not made up of repeated verbal tics (“You know,” “like,” “so I mean,” “I was all,” etc.) but are much more nuanced than that. They are not real-life dialogue. They give the impression of a real-life voice while remaining a unique construct.
Transportation: A good voice envelops the reader within the world of a book. It puts us in a certain frame of mind and lets us see the world through someone else’s perspective providing not just the details of that world but giving us a sense of the character of the world. Basically: see J.K. Rowling.
Authority: From Bryan Russell (aka Ink) (full comment below): “For me, one of the absolutely key elements of voice is authority. With a great voice you know the writer is in control, so in control that the writer vanishes and you see only the story… A great voice carries you through the story, compels you through the story. I think all great voices have that… There’s a sureness to a great voice. The words are simply right and the rhythms of the prose are buoyant. You won’t sink, not with these voices.”