Public good is a nebulous concept, but it is something that each of the folks who sign up as mentors judges independently when they are rating the projects and talking about them.
Most of the folks who are offering to mentor have been involved in the community for quite some time and have a pretty good overview of what is going on, and what are currently active pain points.
With 25 mentors we get a pretty good cross section of the community. We aren't really able to canvas outside of the mentor group during the approval process by google's guidelines, since we shouldn't leak information about unaccepted projects.
Something like that uservoice site might be used to gauge public opinion of general ideas before the proposals start coming in, but in the end students write the proposals we get, so the things we would have polled about are inevitably not quite what we're rating anyways. We rarely get something that is just cut and pasted from the ideas list. Consequently a generic rating that doesn't take into consideration the actual proposal isn't worth a whole lot, beyond giving students an idea of what might be a successful proposal. There is a lot of variability in the ratings for projects based simply on what we know about the student, how clear the proposal is, and how achievable his or her particular goals are.
In practice, we've been able to make sure that a couple of slots go to separable tasks in projects like cabal, haddock, and ghc that benefit everyone and that exceptional one-off projects don't get shut out completely just by asking each mentor to rate all of the projects, even the ones they aren't interested in mentoring, and from the discussions between the mentors and between the mentors and students that ensue within melange.
My main advice is that if you want to get involved in the process, the easiest way to peel back the curtain is to volunteer to mentor! We're generally quite open to adding new voices to the discussion.
-Edward