Perfection needs three things and two more:
1) An objective and permissive proprietor who only rejects a piece of music if there are legal issues or bad production values. Everything else can be asterisked in any of several ways and, thus, filtered by site users who so desire. A special "professional quality" asterisk can be assigned to any submission that has all the checkmarks.
2) If a community is allowed to develop via forums or threads without proper guidance, it will eventually develop its own customs and attitudes on what constitutes an acceptable music submission. This kind of atmosphere should be prevented. No one should have to jump through hoops of popular opinion to get their music on the site. Peer reviews of works in progress should be prohibited.
3) Music should be sortable by pretty much every relevant variable, such as game, platform, alphabeticality, genre(s), original composer, remixer(s), date original work published, date remix published, original album (where applicable), etc., and also by the meta variables.
4) Accessibility. Not as important as the first three, but it'd be nice if there were primers, outside of the forums, to help newcomers to the remix scene know what they'll need to know to get started...computer hardware and software, external hardware, etc.
5. Reliable, consistent attention by the administrator. No vast periods of silence on the front page. No prolonged periods of downtime. No enormous backlog of submissions.