It is a very good film. But people need to realise that the fact that it is at the top of the IMDB top films list is a reflection of how broadly popular it is amongst the masses. Any critic can point to Citizen Kane as one of the greatest films of all time, but a man on the street may find it quite boring. Seven Samurai might be less appreciated because of the language barrier, or The Godfather for it's length.
When a collaborative effect is made to rank films based on ratings given by individual people, is it any surprise that a film like Shawshank, which is easily accessible and understandable, moving, well acted, written and directed with one of the most feelgood endings in the modern Hollywood era, has risen to the top?
I don't think it is a case of it being overrated, it is just one of the most popular films in recent times.