If you allow people to vote after they have seen the rankings, you have the wisdom of mobs. The only way to get the wisdom of crowds is to ask people before they have seen other people's opinions.
That's an interesting point--why does every site need to quantify for the user the particular score of a comment? Why not indicate popularity by simply ordering the comments?
Is it a trust issue?
Slashdot no longer displays numeric karma and you can disable display of comment scores.
Batman might be the best movie out recently but in 2 years time, it will be a dim and fading memory while the Godfather will retain its appeal as it has already proven that it can.
IMDB just needs a temporal filter to keep the latest trend from skewing the universal result.