It's a nice idea and something we can look into. One possibility would be to create a page plugin for this site, that way it's linked to the users and permissions system. Rating plugins could be limited to users with some points to prevent spam.
The problem is really how to ensure quality? We'd need some people checking and approving submissions, which is time consuming and could require some advanced knowledge (to check a plugin doesn't have malware and so on). Then they would also need to go back and check old submissions in case they disappeared, or don't work in the current Q2A version. I wouldn't be able to spend much time doing that since my time will be mostly spent on the core.
Regarding your 5th point, are you asking for plugins/themes to be sold through the official channel? I think that would be unlikely to happen (and since you linked to Wordpress, note that they do not sell any premium stuff themselves). There are plenty of commercial Wordpress themes/plugins and they do fine on separate sites.