What makes The Shawshank Redemption so cinematically important?

I believe it is due to its great acting and very memorable plot.

I'm not aware of any widely-used cinematic techniques that were invented due to the film.

When the film was initially released, it struggled at the box office (it earned just $727,000 in its opening weekend). Despite this it received huge critical acclaim, garnered seven Oscar nominations and the rest was history - it's simply become one of the most popular, beloved films of all time.

Quite why this occurred is due to a number of reasons:

1. The friendship of Andy and Red.
2. The "simplicity" of Andy's escape.
3. The longing for Andy to be free, given his ordeal.
4. The uplifting nature of the finale of the film.
5. The perseverance Andy showed.
6. Morgan Freeman's sensational voice.

It just touches on so many wonderful, magical issues and that has continued to resonate with audiences ever since. As Tim Robbins (i.e. Andy) himself said:

'All I know...is that there isn't a day when I'm not approached about that film - approached by people who say how important that film is to them, who tell me that they've seen it 20, 30, 40 times, and who are just so... thankful.'

"Important"? I'm not sure it's really "important", but it's a great movie. It's a wonderfully crafted revenge story. And it's not a revenge you see coming. It's not obvious like "I Know What You Did Last Summer" or "I Spit On Your Grave". You don't even see it coming until it happens. People love a good revenge story. People want to see the good guy get beaten down repeatedly until he comes back and wins. It's "Rocky" in a prison.

The fact that the actors gave amazing performances also helps quite a bit. Add to that the fact that it was written by Stephen King, and you've got a winner.

From IMDb's own explanation, the following explains how their rankings are determined

How do you calculate the rank of movies and TV shows on the Top 250 Rated Charts?

The following formula is used to calculate the Top Rated 250 titles. This formula provides a true 'Bayesian estimate', which takes into account the number of votes each title has received, minimum votes required to be on the list, and the mean vote for all titles:

weighted rating (WR) = (v ÷ (v+m)) × R + (m ÷ (v+m)) × C

Where:

R = average for the movie (mean) = (Rating) v = number of votes for the movie = (votes) m = minimum votes required to be listed in the Top 250 (currently 25,000) C = the mean vote across the whole report

Please be aware that the Top Rated Movies Chart only includes theatrical features: shorts, TV movies, miniseries and documentaries are not included in the Top Rated Movies Chart. The Top Rated TV Shows Chart includes TV Series, but not TV episodes or Movies.

It is essentially based on highest average ratings, taking into account the number of ratings.

EDIT: Due to the clarification of the question I'm adding some more to my answer to describe my personal view of what makes the movie such an acclaimed classic.

In a sense, the movie is a perfect storm of elements that make it such a compelling and memorable movie.

1. Written by Stephen King who is and was at the time considered one of the greatest American authors of our time.
2. Period piece depicting life in an American prison in the early 1900s.
3. Classic story of a man wrongly accused gaining his revenge and redemption.
4. Great twist including an elaborate and fantastical prison escape a la Count of Monte Cristo.
5. Compelling and sympathetic characters.
6. The hero wins in the end due to a combination of book and street smarts.
7. Wonderful casting and acting

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.

The movie garnering so much acclaim may be due to:

1. The acting skills of the talented actors: Tim Robbins, and Morgan Freeman.

2. Definitely the story is one of the strongest points in the movie.

   Where a normal man, patiently handles wisely all the pressures inside the Prison. A man who is respected by the majority of the inmates.

3. I would like to say that the story of this prisoner whose determination earns his freedom, spans 22 years. It ends with a great result and pays him off. A major suspense which most of us wouldn't think of doing.

And the friendship which they Andy and Red both shared was just an inspiration for all.

It may not be the direct answer, but the movie really had the VALUES and a THINKING what a common MAN can do if he wills to. (Anything is possible with HARDWORK and DEDICATION)

One other thing that's mentioned but understated in the answers above is that it was written by Stephen King and released at a time when movies based on Stephen King movies were usually horror movies and frequently bad.

Early King movies had directors like Brian De Palma and Stanley Kubrick attached and were big deals, and there was the occasional movie like Misery that had Oscar-worthy performances in it but when Shawshank Redemption came out there had been this string of crappy Stephen King movies. So to have this one come out with an amazing story, top notch actors and performances, and from Stephen King and not a horror movie, was really significant.

The movie came from a novella called Rita Hayworth and the Shawshank Redemption which was one of four stories contained in a book called Different Seasons which was notable at the time because three of the four stories were not horror stories, which was unusual for King. Another story in the book, The Body, was turned into the movie Stand By Me, so it wasn't even the first non-horror movie based on a King work but both movies sort of downplayed the King connection originally for that reason.

Trivia: the other non-horror story the book, Apt Pupil, was also turned into a movie.

Very few people have an experience of prison life, And this movie provides such a lovable and at the same time, horrifying depiction of prison life. It's just that people love to see things they can never experience in real life. And it's brilliant filmmaking makes it the most loved movie of all time.