The amount of low quality posts is 19,726 and rising.
This is just a heuristic, it doesn't DEFINITIVELY mean that you have that many low quality posts. So I think you're looking at this the wrong way from the outset.
20% of the questions are unanswered
Again, remember that our definition of unanswered is EXTREMELY strict, so this isn't necessarily a problem. It's better to distinguish between "zero answers" and "answers, but none upvoted" when talking about this with some precision.
This is a really broad kitchen sink question so it isn't answerable, but I'd say the best advice to follow is the same advice I give on every site. Sure, cleaning house is great, but focus most of all on getting new content and new users into shape!
Share great questions and answers
http://blog.stackoverflow.com/2010/09/announcer-booster-and-publicist-badges/Vote, vote, vote
http://blog.stackoverflow.com/2010/10/vote-early-vote-often/
http://superuser.com/users?tab=votershttps://superuser.com/users?tab=votersLove and reward your new users for being awesome!
http://superuser.com/reviewhttps://superuser.com/review
http://superuser.com/usershttps://superuser.com/usersTry to attract experts to the site by helping them get answers, too:
http://blog.stackoverflow.com/2011/04/helping-the-experts-get-answers/
TL;DR -- what matters is not so much where you are, but where you are going -- use the hell out of the /review route.
