I'm pretty liberal with what I consider a "useful" answer. Even if it is not terribly "clear" or obviously shows "research" I figure that if it is worth my time to answer or even just comment then that was "useful" and it is worth an upvote.
As for using this meta site, I'm not sure how your post is a question, but as far as an actionable answer to "how to get people more engaged" in a way that is not just an opinion, upvoting and politely encouraging new users helps a lot. The SE site philosophy is not your average forum structure, but once people get hip to it, it is (obviously) a very useful resource... so it takes time.
There's no real way to discourage drive by participants and one time question askers... but even minimal participation can be useful. What I think drives people away, however, is rudeness and arrogance in answers - especially for those who do not speak English as their first language. It doesn't seem so bad here, but some SE sites are a little ...judgemental. All in all tho - I think that there's just a learning curve that will always be a factor, but the best thing to do is encourage excellence and be helpful - like editing posts and answers for clarity and keeping the focus on verifiable information.