Edit: Posted a similar question on Area 51.
It seems that there is only one part keeping us in beta: our lack of asked questions. Here's a screenshot of this (as of August 14th):
I know this isn't really our culture, we prefer to have more thought-out questions than a simple Q&A question on our main site, but how we can get some more people to come up with their own challenges? That could probably alone get a lot of progress in that direction.
And, by the way, I know of the similar question here, but in the span of that 7 1/2 months, everything has increased except for the questions/day. The "avid users", "total users", and "visits/day" have increased about 30-50% in that time period. Questions/day decreased by about 1 per day.
It also appears that Code Review has a similar problem with their answer ratio (1.8, with everything else better than ours), because of their culture. They are the only one who's been in beta longer than us, and not by long.
As a side note, at our current rate of growth, we would rival Ask Ubuntu in size by the time we would get that required number of questions (15) a day.