Two characteristics come to mind:
Indomitable Spirit and
Integrity.
Sticking around the site, even when you don't like the harsh implications
of some things. And having the integrity to admit you don't know
everything and being willing to share corrections of your facts.
I win some debates and I lose some. But my goal is to discover what
the facts are and not to cling to being right. So to me being an Expert
is being able to admit when the facts have changed and be able to
share that new knowledge with others. Many people get stuck in
denial of new trends and that in itself prevents them from absorbing
what one might call expertise.
I'm not saying the expert thing is anything too important, but to me it
implies the person knows the current arguments and will focus on new
facts when they learn the facts have changed, even if it means that
sometimes they have to admit they were wrong.