What particular factors might have begun to inhibit participation in 2006, when we know it began to stagnate?

There's a lot of useful information in the wikistats in terms of cross-language comparisons, and there isn't much analysis that I'm aware of. Just from an initial eye-ball estimate at a few of the top languages, there does indeed seem to be a pattern of new article rate peaking around 6-18 months before editing rate and active contributors peak.

When I get a chance, I'll try to plot total articles against active contributors for a bunch of languages, which should give some indication of the extent to which the opportunity to start new articles (probably the most important form of low-hanging fruit) is what draws people into the community.--ragesoss 20:35, 5 November 2009 (UTC)

While this pattern is interesting, we should avoid jumping to the conclusion that the theoretical explanation for it that lead to this examination of the data (e.g. "most of the articles that most people wanted to write about had already been created") is in fact the most significant cause of the pattern. We should probably cross-reference with other data like number of people who are participating regularly, number of people with admin privs, number of people "touching" each article (or debating them on the talk pages), and rate of article deletion. It may be the case that a slow-down in article creation doesn't indicate a slow-down in the number of interesting topics people want to introduce to wikipedia, but rather that topics get harder to introduce due to an "entrenched" community watching and reacting to article production, and people get discouraged from trying to introduce more obscure topics that are not part of the knowledgespace of the existing editor/admin society (or even non-obscure topics) because they have less time to fiddle with an article all on their own, they start suffering cross-editing activity that requires merging, they get into debates/wars with other editors who have a different vision for the article, or their work (or whole article) gets deleted or tagged as needing improvement.

(The tags identifying how an article needs improvement are supposed to encourage good articles and more work on the articles, but I suspect that newbie editors find them challenging--not always in a fun way--and possibly discouraging.)

Yeah, I don't think we'd want to make a specific conclusion like "articles are already created, and people ran out of things to contribute". But we could probably make a broad conclusion that there is a natural slowdown, and that it's not based on idiosyncrasies like the introduction of flagged revisions, or changing the privileges for anonymous editors. There are things that happen as a mass of people develops into a community with a culture, and we need to create a better on-ramp for new users so they can mingle and embrace that culture.

I am the antithesis of your average Wikipedian: female, 56, did not finish 10th grade and an autodidact. I also consider the concept and execution of "political correctness" to be obscene.

Bluntly then:

• Wikipedia is top heavy with policies and guidelines, in language that is NOT easily comprehensible; it discourages participation from the get-go. In other words WP has become "Nerdsville". (And no offense to nerds intended, either!) A major overhaul and condensation is indicated.
• Advertise in language that is short, sweet and to the point. Run banners that change every minute of the day:

Got the info you wanted? Tell us something you know!

If you learned something useful - leave some knowledge behind!

Know something interesting? Put it here!

People like you make the difference! Join Wikipedia today!

Everyone knows something important. Share it with us! etc.

• In business an owner will almost always choose a manager in their own image, i.e. someone they understand and can relate to (or crudely: 'Fish always stinks from the head'). WikiMedia need to bring an iconoclast or two aboard.
• Finally: get a professional trouble-shooter with a proven track-record to analyze the problem. "You get what you pay for."