On the face of it it's shouldn't be that great a film, I mean the plot 'twist' is largely predictable, and like most prison dramas, it's not short of a cliche or two.
I think having Morgan Freeman doing narration was a masterstroke though, it just kind of washes over you and sucks you in. He has the gravitas to convince you that he really is Red reminiscing about his time at Shawshank.