Via proposals on Brainstorm usually. People vote there on ideas for changes, which may or may not get picked up.

Noise in the forum itself usually doesn't get very far unless someone in the community picks up on an idea and runs with it themselves.

I may be wrong here, but for a proposed change to the whole structure of the site, or a new part of the site, it would need to go through Nokia/Reggie/The Community Council.