What makes a movie great?

I recently watched On the Waterfront, to knock off a film from the list of films I haven't seen, and help put me in the mood for the upcoming 100 Greatest Films list. When writing the entry for the film afterwards, I realized something. Having to look up the plot wasn't my sole deficiency. In On the Waterfront's case, I could decipher the plot to some extent, but I didn't really know what it was that made it so great. I realized that I was too little of a film buff to know what made certain films great, how they served as an influence on the medium.

Then I realized something else. Something worse. Some films did not influence people to come after them. Some films aren't even useful to study for their art. They're just considered great, entertaining films. Professional film critics would be hard-pressed to say what exactly makes them great; I would be woefully underqualified. And I certainly don't know how to analyze the art of a film, even if I have watched it.

The AFI often brings in luminaries in film to comment on the films on its lists, and often all they say is how much they like the film, or particular parts of it. Several lists are accompanied by brief blurbs on the films, but they don't do them justice. I want to take some time to explain the films and why they are so often considered among the greatest of all time, but I don't know one lick about them. I'll probably turn over many films' entries to others (remember, I'm taking applications!) But that doesn't make me look very good.

So, I'm turning this over to you. What is it that makes a film, and specificially the films so often mentioned among the greatest of all time, great? What should a layman like me who knows nothing except the films he's seen recently know about the films he's probably only heard of through the various greatest films' lists but should know in greater detail? Feel free to leave a comment to this post.