Thanks for the replies. I agree @frissdiegurke that it is a social network feature. Essentially I am looking for another way of adding reputation. I.e. the current one shows how helpful, accurate, ... a person is. Now suppose that this forum is also a media for people to buy stuff. Now, we also need to rate sellers based on their quality of service.

Is it still doable by a plugin, or do I need to change the core?

A more specific role management system that lets the administrator define moderators and assign them to a specific thread. Also, I want to decorate these moderators (i.e. adding somthing to their name, or add a icon, etc).

Is there a plugin that enables the user to rate and comment about other users.
If there is not such a plugin, where to I start to add such a feature?