I'm not very worried about an endless September. LW is pretty good at downvoting people when they make rookie mistakes in reasoning and argument, or when they are mean or trollish. The new troll toll (or whatever solution we settle into after a few months) should go even further toward preventing endless September. Moreover, I think the content itself here does a fairly good job of filtering out many kinds of people we don't want.

Finally, I think Xachariah's point is important: "the eternal September effect is primarily caused by new-member with new-member interaction." I would say that LW already does a good job of limiting this. For example, new members who don't understand the culture are downvoted, which means their comments are hidden by default. Also, people are already incentivized to lurk for quite a while before commenting or posting, because the community is clearly intelligent and is constantly using community jargon they could easily be downvoted for misunderstanding.

I also don't think we should make it harder for people to join (e.g. with a quiz). Instead, I think we should make it even easier for the kinds of people we want to find LW and engage. Here are some ideas for doing that:

1. Improve the wiki (already in progress).
2. Create a nicely-formatted ebook of The Sequences (already in progress).
3. Have an army of volunteers regularly comment on selected blogs and discussion forums (e.g. for computer scientists, cognitive scientists, mathematicians, and formal philosophers), linking back to the stickest relevant LW posts. (This project is under development: if you want to be part of this once it's ready, please notify malo@intelligence.org.)
4. Create an attractive "Welcome to Less Wrong" page that can quickly guide people to posts they're most likely to be interested in. This would include a video of somebody explaining in 5-10 minutes what Less Wrong is about, the values of the community, etc. (Project under development.)
5. Create a "community values" page. I like this idea of yours very much. The values could be pretty simple, things like: (1) Don't be a dick. "Do unto others 20% better than you expect them to do unto you, to correct for subjective error." (2) When disagreeing, aim for the higher levels of the disagreement hierarchy. (3) Be clear. Be specific. Give examples. Hyperlink to longer explanations or relevant articles. (4) Before commenting, try to list some reasons you might be wrong about the thing you're about to say. (5)...
6. Have the system deliver a welcome message to a user's inbox when they first sign up, one linking to the "community values" page, the "welcome to LW" page, etc.
7. Create handy one-stop FAQs (on rationality, decision theory, etc.) that become standard resources for their subjects on the internet, and make sure they link heavily to LW. (This project is also in progress.)

I don't think I like the idea of splitting the community into hard and easy forums, nor the idea of inducting something like Crocker's rules into the code of conduct.