I think that the way stackoverflow with the reputation system is really good. For a private project I would like to create a website with forum / profiles / ... that also uses a kind of reputation system. In this way people can only do several things (creating items in lists, ...) if they already have some reputation points.
Reading things about buddypress shows that it could be the right platform for this project.
Is there already something like the reputation system in the buddypress core or via extension available?
Does buddypress support subdomains?
The http://wordpress.org/extend/plugins/membership/ membership extensions looks really good. The difference for me would be that the membership depends on the reputation points. Unfortunately the required full membership extension costs to much for a private project and the free version only supports two membership status. Is there something equal?