Shawshank - explain, please...

I've been meaning to watch Shawshank Redemption for years, and finally got round to it last night. Now I have to ask - what was so special about it? Seriously, I just don't understand. I've heard people talking about it as a classic, utterly brilliant film, not to be missed, etc etc. And I thought it was ok, it held my attention but didn't engage my emotions at all. Or at least not as much as I'd expected.

The violence and the injustice stung, of course - but I'd been much more moved by the Green Mile, and a hundred times more invested in Cool Hand Luke.

So I'm left wondering - why do people love it so much/rate it so highly? What did I miss?