I did a Google search for a 'report post WordPress plugin' and the only ones I found were 2 years out of date. I feel that there is a need for such a plugin, especially if it would work with both multisite and single site installations (ie. reporting guest posts/custom posts against the rules for single sites).
Ideally the plugin would show a simple 'Report' button on posts (configurable by the settings to limit certain types if the admin wants it that way) which when clicked, brought up a report form for the reporter of the abuse to fill out. The admin would receive an email when a report was filled out with a link to the reported post in the email.
What do you folks think?