I believe people dog pile on top of the movie because they have nothing better to do than join a group mentality that "this is bad because others said so."
The movie came out after Costner's giant post apocalyptic flop Water World, and that is what stains this movie. It's a preconceived notion that this is also bad. Then on top of that, The Postman came out the same time as The Titanic. It didn't do well in the box office because of that mega hit it contended with.
The movie is nowhere near perfect, but it's not as bad as the singularly minded Internet trolls will lead you to believe.
If I were to grade the film, I'd give it a B average. There are so many worse things to watch out there.