1) Film taste is far too subjective for it ever to be possible to decide which list has more merit than another. Except for this list. This list is better than your list.
2) How can you compare, say, bad films, when your decision criteria are so varying for different kinds of films? Eg. Balls of Fury made me laugh more, but it’s not like The Kite Runner, with its child rape, is meant to be funny.
3) They are too rigid for a constantly changing film landscape, the need to modify is too much. Film-viewing loses its enjoyment if your first thought afterwards is “oh man, I gotta go update my blog list with this!”
4) You’ll invariably come across as a more annoying version of Barry in High Fidelity. And nobody wants to be a more annoying Jack Black.
5) You’ll always wonder, when looking at other lists, who just missed the cut. Who cares about Film4’s 50 Films to Watch Before You Die? What if I live a day longer than expected – what’s the 51st film I should watch?!