Honesty.
Non-fiction, to me, is not the place for hyperbole and widespread generalizations. I've read a few memoir type books over the last few years and some were good, some were more interested in selling me on the author's brand than they were anything else.
There is an element of the personal, and the passionate, I think, that needs to be in non-fiction because without that, the writing becomes dry. Depending on the content of the piece, though, there might be an inevitability of dryness.
When it comes to specific topics, being the kid in the corner talking about something with full animation of face and hand gestures is the way I want to be as a writer. I want to be able to support myself with facts, though, and not rely purely on passion.