A bulleted list of answers others have written:
- Generates a new insight (TurnTrout)
- Is good for something (adamzerner)
- Shows its work (bvxn)
- Ties up its loose ends (curi)
- Resolves a disagreement (curi)
- Shows effort (Alexei)
- Well-written (Alexei)
- Surprising (Alexei)
- Credible (Alexei)
- Summarize work (me)
And certain topical interests which LW is a topos for:
I'm throwing in that I like posts and comments that compress knowledge (such as this).
My further two cents are that what people answer here will be somewhat unrepresentative. The answers will be a certain set of ideal practices which your answerers may not actually implement and even if they did, they might not represent the community at large. The honest answer to your question is probably data-driven; by scraping the site you could generate a better predictive model of what content actually gets upvotes than people will tell you here.
But nevertheless there is value to your question. The idealized picture you'll get is in fact the picture of the ideal you want. If you take onboard people's best-case answers, you'll make stuff that the most engaged people want the most of, and that will contribute to a making better community overall.