A mockup not very well thought through but you get the basics example I give below:

If you think this is a good idea spread the word. I know it might seem like a simple idea but I think it could be very effective and helpful to a growing number of people. I picture it as not being dissimilar to the way Amazon.com handles their recommendations system. If only they could add that to an optional part of IMDB.com or make a separate niche site. Anyways if you like the idea Digg the story and hopefully the collaborators that be will put this into motion.
What you would need is a Wiki style database of all TV shows and Movies and a tick box whether you've seen it or not with an optional episode rating next to it to gauge episode popularity. People from countries around the world should participate to make local content up to date as well.
View the 2011 updated edition of this post with solutions to the problems I presented in this post.