Personally, I think you should just allow all registered users to leave comments/feedback/ratings to start. Let us, the webmasters, state the rules for our sites and deal with any problematic users - it should be clear who is abusing the system...
Don't try and over analyze it from the get go or nothing will ever get done (been there, believe me)
At least this way, we can use it in real-world situations and give more concrete feedback and suggestions.
Cheers,
~Regs.