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How much you contribute to the community/the section you're nominated for (doesn't have to be just threads, but insights on topics or topics for threads like this), how active you are, how well you get along with others, etc. I think those are the basics, from what I've seen. ...I guess attitude can also be a factor, but remember, people have up and down days so I don't think that should be a huge decision maker if you ask me.
And, from a pessimistic person's point of view, it's determined based on how many staff/ex-staff you've befriended! ...Which is not true, of course. I've seen this idea pop up, but it's been disproved so many times.