So: say that a research area has meta-h-index at least h, iff at least h of the researchers in the area each have h-index at least h. This would give some indication of the size of a community, how tight-knit it is, and how heavily people in that community tend to cite other work.
I wonder what this would tell us, e.g. about different subcommunities of the algorithms and theory research areas?
I'd be wary of broader comparisons using this measure, though, e.g. between CS and physics, because different citation patterns between the fields could overwhelm any other differences.
ETA: Suresh suggests normalizing the citation counts to reduce the differences between fields. And points out big differences in citation patterns even within areas of computer science that are close enough to theory for him and others to work in both.