This plugin has great potential and could fill a gap that I think exists in GForum - providing the ability for users to help 'police' their own communities. In large forums with thousands of posts per day providing some way to alert mods/admins about abuse prevents them from having to troll through every post on the forums every day. I think to make this really valuable you need to provide some kind of interface that mods can access as well. Probably not unlike a standard ticketing interface. I would suggest the emphasis should be on reporting posts, not users:
1) User reports a post by clicking on the link and adding a short message
2) All moderators of the forum that receives a PM that a user reported a post (can switch this on/off)
3) Mods can go to the 'ticketing' interface to see a list of all reports
4) If a mod chooses to deal with the issue he/she can assign the ticket to themselves.
5) Mod deals with the issue and enters his/her notes about how it was resolved
6) Mod closes the 'ticket'.
7) Retain a searchable historic log of all closed tickets for future reference..
If I had all complaints in my forum come to me as the admin it would drive me nuts!! Just some thoughts to keep in mind if you ever decide to build on this plugin.