I think on things like IMDB, there's a definite degree of "accessibility". Shawshank always does well because it a very watch-able film, everything about it is "good". It never has pretences to "art", it never steps into the bounds of fantasy (which The Green Mile does). It's just an incredibly solidly made film, telling an incredibly solid story, with incredibly solid acting and compellingly solid characters.
It's a film that everyone can enjoy easily. Is it the best film ever? No. It's the one film that almost everyone who has seen it will agree is a good film though. I guess it's the best quality vanilla ever
I always watch it when it's on (even though I own the film). Jurassic Park is the same (though I think Jurassic Park is massively flawed - it's just so entertaining still) and I think that's important; a good film is one you can watch over and over.
A great one though can change the way you view the world, or films , forever.
Tom mentions Wall-E above. I love Wall-E until he leaves Earth, then I don't give a stuff. Until that point, it's a gloriously romantic film. The character of the film is sublime, it's just a wonderful, emotive piece of film-making. It's that "art" thing though - watching a film just for the emotions it conjures in your mind rather than the story it's telling. If you watch Eraserhead or The Seventh Seal (more so the former than the latter mind), it's about trying to see the world through the eyes of the artist and it's open to interpretation and what you get from the film is entirely subjective.
I like those kinds of films too, but they're much harder to get a fix on. I think Seventh Seal is a better film than Shawshank for instance, but I would want to watch it all the time, and I certainly wouldn't expect others to love it.
I agree with Tom about Tarantino films to a degree. I think they're overrated, but I still think they're good. There's a particular way you need to watch the films to have them work. They don't often tell a "complete story", but are rather a collection of scenes that vaguely fit together. They can be hard films to enjoy because they just don't work along normal film templates. They don't fit. I think that quite often, people mistake for being really clever when it's just a different approach. Inception does something similar, it fools the viewer into thinking the film is much cleverer than it actually is. When you break through that though, Tarantino films are still excellent (where Inception is a bit meh). His character interactions/dialogue are witty, cutting, sharp and brilliant. His "big scenes" are always made to perfection; the use of music, shock, character or ambience is always spot on. Often they kind of stick out a little from the rest of the film, but they're meant to. Tarantino gives us memorable moments, holiday snapshots that convey the best of the cinematic vacation we've just been on.
I've already mentioned one film I find overrated, Inception. I was wowed at the cinema, then it slowly sunk in I'd been conned. A second watch was proof. It uses clever techniques and misdirection to fool you into thinking it's much deeper and cleverer than it really is. In reality, it's very simple with a "twist" to make you go "oooo" and think you're clever for spotting the twist. It's a well made film, but it's not actually anywhere near as deep as people seem to think.
I'm the same with The Dark Knight. I just don't think it's a brilliant film. I think there's a lot of really great stuff in there, but there are some real extraneous scenes which add nothing but an extra ten or twenty minutes to the film, which is an issue as the film is about twenty minutes too long. The Harvey Dent/Two Face stuff went on for too long and it really needed to be dropped into another film, but I understand in terms of "The Trilogy" why it was important. It just dragged the film on from being sharp and exciting to "bum numbing".
