Funny you say that, I think Shawshank Redemption is so overrated that I want to puke. I'm not saying it's a bad film that would be a false statement, it is however grossly overrated and I would never in my life consider it one of the greatest films of all time...
For me the emotional impact of the story in Shawshank Redemetion is so powerful and stirs such strong emotions that the film is indeed worthy of it's high praise.
It all depends on what one wants from a film.