To tie ratings to email addresses would be a very strange way of solving this problem. I fully agree that something like this is needed though. It's been discussed many times before.
With a multi client and multi user setup, there is lots of things that should be user or group based. Like play counters, skip counters, last played, ratings and so on. This should be user dependent. But instead of using mail addresses, it would be far better to introduce a real Access Control List system. A place where you can add users, give each user read, write and delete rights to different type of media and settings. I think that for a starter, the media should just be divided into Video, Audio, Images and Data. So you can select what users can access (view), write or remove this media. Write should be divided into real tagging of the files, and the normal statistic counters etc, that is filled automatically. There should also be a way of disabling users to access options and edit view schemes. You should also have settings in this ACL, to be able to lock certain users to Theater view only. This also means that you need a roller option in Theater View, to change users. Select user, and use on screen keyboard to insert password. Pin code only, with a number pad might be quicker and better. Introduce gestures for Touch Screen users perhaps? Switching users also have to be done in standard view of-course.
