Compare Unwatched Films with friends
Solve the problem of "What haven't you seen?"
Two friends wander the store but can never find something they both haven't seen. A list of films filtered as unwatched for two users could be sorted by popularity and save the day.
From this point it shouldn't be too hard to expand the formula to deal with the multiple roommate or group movie night problem. Trying to find a film that you and 5 of your friends haven't seen would wonderful!
The best option we have at this time is to visit the Watchlist of a friend and use the “Hide Watched” filter to only show films that you haven’t seen. Obviously this doesn’t work for multiple people!