I imagine that like the per-site-metas of the Stack Exchange 2.0 sites, the reputation on the Area 51 Discussion is non-existent.
And to a degree, this can be seen as even more intentional and desired. The point of the Discussion is not to have a handful of "Meta Area 51" users who police the place and pass judgment on every proposal. It is meant for the followers and committers of each proposal to have some medium in order to communicate points. It is for those invested not merely in the Stack Exchange Network, but in the individual proposals themselves, to shuffle the ideas of proposals into real sites.
And so, the benefits of participating in Discussion is not that much about some global number that you can use as a reference point as you dance with other discussion threads. Instead, it's getting known with the future community for the proposal if it progresses, which helps build a stronger community when the site eventually reaches Beta and has its own per-site-meta. Of course, you still won't get reputation there, though. ♪