This is interesting - but I agree that it is absolutely essential to have the option to switch between Wordpress.org and GitHub. I also agree there needs to be a good way to limit the results of the search. Even though Wordpress.org is not a perfect model for ensuring plugins follow coding guidelines, there is a system that keeps out most of the chaff. I would like to see them jettison plugins that have not been updated in years(!) - but the same is true on Github. There are going to be orphans that no longer have an active community that turn up in a search if there isn't a way to set criteria for your search. Yes, you should check the details before you install anything from anywhere - but still - setting criteria would lower the overhead in search.

Actually - I guess I'm preaching to the choir on the criteria issue - that should be a core feature of plugin and theme search on the admin dashboard.