I wrote out some of my reasons why I don’t think a count of how many people have marked an observation as reviewed would tell us anything meaningful here.
(If people were more consistent about marking life stage for insects, you could simply use this as a filter instead of marking caterpillars as reviewed.)
The url snippet for excluding observations ID’d or reviewed by a particular user is the best concept I have come up with to help IDers avoid duplicating efforts (e.g., if user A has ID’d the observation I don’t need to prioritize looking at it). This does require some extra work from IDers (either bookmarking/using a link or manually changing the url each time) and I believe this doesn’t work as an “or” query (i.e., observations ID’d by either user A or user B), which limits its usefulness unless there is some additional coordination (dividing up by regions, with one or two users who have reviewed a significant portion of the total). But it seems like a starting point for organizing a coordinated clean-up effort.