I'm also running a question & answer community similar to this one, but about personal finance. Here are some of my thoughts on the subject of gathering the critical mass you speak of:
First, I agree with Elie 100% on the 'fake' users and the trial beta concepts:
I can confirm through my own experience that having friends and family helping out is invaluable.
I also post simple FAQs using supplemental test accounts, and that's been working out well to draw in search traffic I wouldn't have otherwise got if I waited for such questions to be asked organically.
I've also made it very obvious that my site recently launched: I have the word BETA prominently displayed next to the logo.
But having some good initial content to bootstrap isn't sufficient by itself. First, your content ought to be indexed by the major search engines (at least Google.) Second, people arriving via search need to find what they're looking for when they get to your site. i.e. the content needs to be useful: help somebody solve a problem, answers a question, etc. If your content is useful, you've got better odds of converting a search visitor into a user that'll come again. If somebody just finds matching keywords at your site, i.e. lots of words yet little substance, then they are likely to bounce.
What's also been working out for me is participating at similar sites – as I do here. While my username is obviously an ad of sorts, I do try to post answers and comments that genuinely add value to a conversation. I think this has resulted in substantial visits to my site. (But the competition is catching on to this strategy!)
Note that I didn't have a popular blog or other site of my own that I could leverage directly, such as Dharmesh has been able to do here. I know now, though, that laying such groundwork well in advance of a venture would have been useful! Still, I don't believe that having a successful blog to leverage is a guarantee of success ... it would just have been a big help. I believe strategic thinking and hard work can more than make up for it.