Via Lawyers, Guns and Money, I see that Roger Ebert has tackled the question of why "The Shawshank Redemption" has the highest ratings of any movie on IMDB.  Ebert makes somes interesting points about the appeal of "Shawshank", but in this enthusiasm for talking about what makes the movie so good, he misses talking about what it has that would make it rise above the ranks of other movies that have similar qualities. I would argue there's two reasons that "Shawshank" rises above all others:

1) It's got no female characters.

2) Mediocrity rules.

I actually feel bad dinging the movie for these, because in a way, it's unfair.  "Shawshank" isn't a mediocre movie---the acting, the script, the directing are all pretty damn good.  But it still rises to the top for the same reason that mediocrity rules.  It plays it safe.  And not having female characters in it is one of the most important ways it plays it safe.

The idea behind "mediocrity rules" is that true greatness always runs the risk of offense, or at least turning people off.  For one thing, greatness is innovative, which means that you lose huge portions of the audience that wants a warm bath of not being challenged at all.  Think of the film that deserved the Oscar that year, but lost out (along with "Shawshank") to "Forrest Gump"---"Pulp Fiction".  Nowadays I don't think "Pulp Fiction" perturbs too many people, but at the time the film was so shockingly different that it didn't have a chance in hell.  In fact, Tarantino is still doing one movie after another that offends the typical sensibilities of the mainstream American audience (including, which I'll get to in a minute, the shut-up-ladies phenomenon), and still not getting loaded down with Oscars, in a fashion that will be seen in the future as a scandalous oversight.  "Pulp Fiction" was rock and roll, and "Forrest Gump" was Michael Bolton.  "Shawshank" was a Frank Sinatra album---solid, classic even, but not great.

"Shawshank" doesn't have anything in it that's going to chase people out the door.  It appeals to the smart and the stupid alike, the liberal and the conservative.  Everyone can get behind the story of a man redeeming himself after the system grinds him down unfairly.  It's set in the past and outside of the world, minimizing the chance of referring to anything that triggers people's distaste.  There's nothing polarizing about it.  If it was a blog comment, it would get a lot of upvotes, but no one would bother to downvote it.  Functionally, this is what happened to it on IMDB.

It also benefits from everyone having seen it.  And the reason is that it, like "Back to the Future", is on cable non-stop.  I'd bet it's playing on a channel somewhere right now.  But the reason that it is has everything to do with these principles.  It seems like the safe movie you can play on cable non-stop.  Everyone likes it and no one hates it.

I would argue that its lack of any real female characters contributes to the feeling that it's safe.  As much as it pains me to say so, I think that female characters are polarizing.  Hollywood's preference is to have female characters with no internal lives, no physical flaws, and no concerns outside of man-pleasing, because they believe (with some reason) that large chunks of the audience find anything else from female characters threatening.  But you also have a large part of the audience---and a growing one---that's frustrated with the lack of understanding that women are people in Hollywood films, and won't like movies that portray women as cardboard characters or dumb bunnies.  "Shawshank" sidesteps the problem entirely by simply not having female characters, outside of a wife character who I don't even recall having any lines before she's killed. 

Women are polarizing figures in our society in general, because of the eternal rule of the patriarchy that a woman is never doing anything right.  Everyone is eager to tear at women and judge women and examine women closely for perceived slights against what they personally believe a woman should be like. There's also the feminist urge to examine women closely to see if they're rising above the gender trap. Simply by being Other, women capture attention and controversy.  There's a reason that Hillary Clinton and Sarah Palin both are more polarizing figures than their male counterparts.  Putting a woman onscreen causes the audience to start dividing against itself.  But "Shawshank" is a bunch of dudes.  This contributes to the non-challenging aspect of it.  Even the rape somehow gets removed from the toxic gender norms that create rape (and therefore allow men to become victims) by the magic lady erasure of the movie.

I make it sound like I don't like "Shawshank", but I actually do.  It's solid.  It's a better quality than most movies that use these strategies to be non-controversial, which is why I think it rises above those movies in the rankings.  But it also falls short of being as good as movies that choose one section of their audience over another.  Tarantino, as I mentioned previously, has moved into giving the finger to people who are alienated by having to remember that women are people when they're watching movies.  His best characters have been women since "Pulp Fiction" gave him license to do whatever he wants.  (And arguably, the most fun character in "Pulp Fiction" is Mrs. Wallace.)  This, I suspect, contributes to the polarizing nature of his work.

Maybe I should call it the "Shawshank principle" instead of, as I usually do, "mediocrity rules", in order to encompass actual works of quality that still avoid giving any kind of offense and therefore rise to the top by default. 

The Shawshank principle, by the way, is why I'm convinced that Tim Pawlenty is going to win the Republican nomination.  People continue to make the mistake of looking at popularity contests as a matter of who has the most positive qualities that draw people to you.  Instead, you should look to see if someone has a polarizing quality that's going to cause them to get some downvotes, in the internet parlance.  This goes doubly so for Republicans, because the whole point of being conservative is being reactionary, rejecting out of hand things that challenge or perturb you.  Liberals can often get swept up into enthusiasm for a candidate instead of simply picking one by process of elimination, which is in part how Obama won. 

Almost every candidate in the Republican field has something about them that a portion of Republican voters really dislike.  Romney will get killed on the Mormon thing---the evangelicals just aren't ready yet.  Haley Barbour is out because the racism thing is too toxic.  (This is a really good sign of progress in our country, by the way.)  Sarah Palin is not only out, but I think women in general are a failed experiment for Republicans, and the glass ceiling is thicker than ever for them.  Huckabee turns off the large portion of Republican voters who aren't evangelical Christians.  (Believe me, the contempt that more secular conservatives have for their Bible-thumping comrades is something you don't want to ignore.)  Trump is just kidding, but even if he wasn't, he's never going to get the Christian vote ever.  Pawlenty has nothing overtly offensive about him. The Christian right doesn't suspect he's a secret liberal or that he's not a real Christian, and the more secular right doesn't think he's a wild-eyed Bible thumper.  He wins by default.