We should add links to important meta Q&As, either existing ones or new ones that we create specifically for the purpose of educating new users
This is partly inspired by what Buddhism.SE has done (based on the comment by Andrew T.); we should add a list of meta Q&As about topics that we feel new users should be made aware of, but aren't currently covered by the tour or the help center. I'm undecided as to whether we want to link to existing Q&As that weren't written for that purpose, or whether we should write new Q&As write for new users as our intended audience.
Here are my suggestions of what I think those topics should be (note that I'm going to assume that upvotes to my answer here represent that people think this kind of thing is a good idea, even if you disagree with some of these suggestions; see the section at the end for more on this):
We have been struggling to find a way to signal to new users that this policy exists, and whilst we can't do so on the Ask a Question page, we can at least put it on the front page of our help center (even though this won't help them until after they've already asked their first question); I think this should go first in the list on our help page.
"Closed" doesn't mean "banned forever", but more like "work in progress"
"Closed" can seem rather final to users who are more used to forums, so explaining to them that it's more like a "work in progress" state and that the community can reopen any closed question might help to make it feel less final, and therefore more likely to engage and work with us to get it into a more on-topic state.
We are an edit happy community
To try to avoid situations like this one, it might be worth having something here to explain how here at RPG.SE, we tend to edit posts into shape, so new users who might be rather attached to "their" post should perhaps not take it personally; it's just what we do here.
Answers in answers, comments in comments
Not all of the other Stack Exchange sites are as strict as us on not answering in comments; SciFi.SE does it often and don't seem to view it as a bad thing. Not using answer posts for comments is also something that should be emphasised.
Encouraging users not to accept straight away
This one might be a bit more controversial, and I don't think it matters too much, but based on the accepted answer to this Q&A: Should we encourage users to wait 24 hours before accepting an answer?, it might help to encourage deeper engagement rather than hit-and-run users who we never see again.
If new users are more used to forums where it's comment to say "EDIT: Blah blah" instead of just making the edit, it might be good to point them in this direction to explain why that's not necessary.
Ok, I'm running out of ideas now, but this is the kind of thing I'm suggesting. I'm not sure if all of my suggestions above are that good or relevant, I've tried to order them according to how important I feel they are, so the first two I really think should be there, the middle ones probably ought to be there, but the later ones, I'm not so bothered about...
How should we decide which topics get included?
I'm going to assume that upvotes to my answer here represent that people think this kind of thing is a good idea, but as for exactly what topics should appear on this list, that could be handled via another meta Q&A, where each topic is its own answer. In other words, I'd submit each of the six suggestions I've made above as six separate answers, and the community can vote on them each accordingly. I assume others would submit additional answers to make additional suggestions.
So, if the policy suggestion gets loads of upvotes, that'll be on the list, but if the one about waiting 24h before accepting gets downvoted or a mixed reception, maybe that one doesn't get put on the list. However, as KRyan points out in a comment, this list shouldn't get too long (although we'd need to include a lot of stuff before we reach the level of how large Buddhism.SE's help page is), so if we have, I don't know, 20+ suggestions, we might need to only include to top 20 or something...