I posted what I thought would be a quick thought (turned into an essay...) on chat to try it out, noticing that it wasn't unusual for posts to be days apart. I hadn't reckoned on the 2px spacing between paragraphs... it's unreadable, so in the name of readability and good typesetting I'm posting it here...
Just to drop down a quick thought - a crucial element of this puzzle that doesn't feature enough is the issue of content curation and ways of finding content.
Click around http://stackexchange.com/sites looking for patterns in what's succeeding, what's surviving and what's stagnating. Obviously, proximity to programmer/geek culture is a big factor - look past that.
Another big factor seems to be the scale between sites like Stackoverflow, where problems have google-friendly hooks that people run into head first and then look for info on specifically - error messages, version numbers, function names, etc, and sites like us, History, etc, where there is great interesting content is often in questions people don't realise they should have asked until they see the answers.
(check out the History site by the way - fascinating brilliant stuff, but they're even more stuck struggling to reach out to their constituent community than we are)
There are essentially two views in SE. There's the latest questions feed, for people looking to help and create content, where stuff needing answers is pushed to people who want to help. And there's the view for people looking to consume content - which is google (which serves sites at the SO end fine, for those questions that fit the format). There isn't really any way to follow premium content and be surprised by it, other than wading through everything sorted by recency to get to it.
For us, the problem is exacebated further because a lot of our great content is surprising answers to dull-looking questions people wouldn't think to ask or give time to (until a newbie does for them) because they don't realise they're not doing something the best way. There's no clue in a list page that there's a great 5+ answer lurking in that answered, dull looking 1 vote question about something you do every day where you think your way is perfect.
What we need is some kind of 'Great content' view that is a face of the site that people can follow and get fed the best stuff - and hopefully from this, they'll get hooked and get involved. We've got just enough of a trickle of great content (=great questions + surprisingly great answers to meh questions) to fuel this sustainably, if it can be presented right.
It could be something high-tech like a page, feed, blog, digest or block down the side of the latest questions page highlighting only recent questions where question or answer have 4+ votes. Or it could be something low tech, like a 'Featured' tag that only certain high-rep people can apply a few times a week (then we show friends and colleagues the 'Featured' tag page) or a regular 'Question(s) of the day/week/month' or "Things we've learned this week" in meta or something.
It's USP in the world of things designers can follow would be the accountability, many voices, openness. For things in your specific niche, it's appeal is obvious, for things in other fields (if successful) it's an interesting low-effort insight into what's going on.
To summarise the issue: programmers (and techies, sysadmins, cooks, gamers...) come to find archive content by googling error messages (and logs, ingredients, levels...). Designers (and academics, enthusiasts...) in general discover content by following reliable sources. We've got good content, we need to give it to people in a way they can follow. The latest questions list page is, rightly, designed uncompromisingly for people who want to answer questions and for getting questions answered, not this.
It's not just us who this is an issue for - we just happen to be a site that has clear potential but is also towards the harsh end of this particular sliding scale. Loads of middling sites (engineering, religions, languages, sites like Android/Apple that have more pros but even crazier pro to drive-by ratios...) are somewhere in the middle on this scale, and even some topic areas in the high end sites are affected (best practices in SO, surprising advanced features in SU...). Get it right here, and loads of StackExchange sites will benefit.