What can be said about The Shawshank Redemption that has not been said before? The film speaks for itself with the amount of awards that it has been nominated for, and won.
The film was released with an 'R' rating. Back in 1994 there was quite a bit of voodoo and hand waving from the MPAA for how a film was rated. It was more difficult to work out exactly what caused a film to be considered one rating or another so the film could be edited to fall under a lesser rating. Now studios actively fight ratings because they understand what the effect is of having an 'R' rating, but this was more rare during around the time period when Shawshank was released.
The film initially did suffer quite a bit from having an 'R' rating. This likely affected box office turnout. The silver lining is that the film was easily edited down for content to it could be shown on television.
Besides the rating, this film has all of the elements of a successful drama. It has likable characters such as Andy, and Red that the audience can relate to and rally behind. The audience is able to get a sense of how the main characters are feeling at any specific time. It also contains characters that we can all collectively hate such as Warden Norton and Captain Hadley.
It is difficult to find any flaws in the film. It is not tedious to watch. Events move rather quickly, and there are not any long lulls or non-sequiturs that detract from it. There is a sense of gritty realism that is often hard to capture on film. Tim Robbins' deadpan delivery always hit the mark. Could you imagine this film occasionally breaking into a song and dance routine? That would completely ruin it.
The story has a well written arc with a definite beginning and end. In the end, the protagonists walk into the sunset, and the antagonists get what is coming to them. There is a satisfaction seeing how things work out. There are times which are very dark, and you have no idea what is going to happen next.
There is a certain inherit sadness of the film which shows how easily life can pass you by while you are in prison. In the beginning, there is a constant wanting to get out, but if you are there for too long, you lose touch with the outside world and become institutionalized. What hits home, is that what goes on in this film is still going on every day. There are many people who are caught up in the system that don't really belong there. Because of these elements, this film is timeless, and an instant classic. In another 20 years, this film will be just as good as it is today. That cannot be said for the vast majority of films.