What is most important to me is that my narrator's voice is believable, and that, though it is clearly an absolute fiction, it has the emotional resonance of memoir.
Quite often my narrator or protagonist may be a man, but I'm not sure he's the more interesting character, or if the more complex character isn't the woman.
When the reader and one narrator know something the other narrator does not, the opportunities for suspense and plot development and the shifting of reader sympathies get really interesting.
I've always felt that the traditional novel doesn't give you enough information about the narrator, and I think it's important to know the point of view from which these tales are told: the moral makeup of the teller.
There's something uniquely exhilarating about puzzling together the truth at the hands of an unreliable narrator.