I think compiling a Top 50 balances the need for a sufficient selection of movies, without scaring away too many people. In addition, a Top 100 unfairly punishes all the great movies that people haven't seen yet - I have so many on my "to watch" list that I am reasonably sure would be in my Top 20 let alone my Top 100. Having to award them 0 votes when I am giving say 80 movies (instead of 30) a vote before them is a bit harsh.