A danger for any new Stack Exchange site is to have a population of mostly laymen, and few experts who can ask and answer the difficult questions which the site's survival ultimately depends on.
Physics is suffering from this and experts decided to create their own site. Artificial Intelligence was closed because of this. Something has to be done to address this problem. Otherwise, many other proposals will suffer of the same fate rather than become extremely useful resources like Stack Overflow has become.
Thus, I propose that a text similar to the following is displayed when ones tries to commit to a proposal:
The success of a new site is predicated on the presence of experts on the subject. A too large concentration of laymen early on is detrimental to the website. As such, we only allow [audience] to commit to a proposal. Laymen can join the site after private beta.
The [audience] above would be replaced by what is found in the "Who is this site for?" field that is filled when creating a new site proposal.
The Economics site's commitment message, for example, would ask:
The success of a new site is predicated on the presence of experts on the subject. A too large concentration of laymen early on is detrimental to the website. As such, we only allow economists and graduate-level economics students to commit to a proposal. Laymen can join the site after private beta.
It's not perfect, but it should at least be an improvement.
Language proposals (English-French translation, Spanish Language & Usage, etc.), for example, could talk about level of fluency to hopefully chase away the merely curious and those learning a language. Again, that wouldn't be perfect but it would certainly help.
Also, to confirm, rather clicking on a button, typing would be necessary. Something like:
If you are an expert of the subject, type "I am an expert" in the box below.
Typing is required, over clicking a button, because it forces the user to read the text. Otherwise, it'd probably still be too likely that someone presses "I am an expert" without reading anything.
It might also be a good idea to lengthen the duration of private beta on topics where you'd expect the incoming of a lot of laymen right at the end of private beta.