I'm debating the same thing, though my field in housing policy which it seems everyone is an expert in these days! I can't make it too general, or else I sound like half the talking heads on CNN. But the alternative is to hammer on some detail, which mostly means some administrative element that is not very interesting.
If it helps, this is a link to a sample policy memo from the HKS website - not meant for the admissions process but rather for students to demonstrate how to include citations in a brief policy statement, but it might help give some flavor for the level of specification and formality they are looking for.
http://www.hks.harvard.edu/var/ezp_site ... y-memo.pdf.
Good luck.