I think you're underestimating the value of an existing community like the Ubuntu one. They have an existing community and just now are using a Q&A site, that is a huge advantage compared to us.
I would guess that we have a far greater percentage of visitors from search engines than other SE2 sites. Many other sites are mainly visited by people from other SE sites, those are also much more likely to contribute. Getting people to contribute who have no prior connection to any SE site is much more difficult.
I don't think we can force the issue in any way, we just have to be patient. Even if our conversion rate is bad, the relatively high number of visits provides us with a big pool of users to recruit. We just have to convince them that it is worth the effort, the best way to do that is to have a big archive of high quality content. To get that high quality content we need more people, it might seem that I'm going in circles here but I think we just have to be patient.
It takes some time to really appreciate the advantages of the SE system, I don't really know how to change that. The ability to improve any question is very important to me, but new users most likely won't see the significance of that. The SE sites attract a certain kind of user, but they are also a bit elitist (in a good way), the whole concept discourages users that are not constructive and that don't put any effort into their contributions. I think that this is a good thing, but it also limits our potential user base. I see nothing wrong with just growing organically, without forcing the issue.
And in the end the low conversion rate can be compensated by a high volume of traffic, which is probably easier to achieve.
To get some quantitative data on our conversion rate I created a query for active users with >200 reputation that visited the site in the last 30 days before the data dump. This data is certainly not ideal, but is the only data available to me.
Site active users views
Gaming 348 15k
AskUbuntu 373 16k
English 198 3.5k
Photography 183 2.6k
Tex 189 4.2k
WebApps 199 3.3k
While our conversion rate is worse than that of other SE2.0 sites, in absolute numbers only AskUbuntu is better than us (out of the sites available in the data explorer). Therefore I don't think we really have a problem here, though it would certainly be nice to improve our conversion rate.
I still think that most users for other SE 2.0 sites arrive from the trilogy sites, and that those users are far, far more likely to become active users than people arriving via search engines. But I don't have access to any data that could confirm my hypothesis.