I've seen 50 of the films on this list -- and there about a dozen more I'd like to see. I remember when it first came out,I got into some very heated arguments with my friends about some of the films on the list -- and I admit, I'm impossibly quirky when it comes to movies.
That being said, there are a few films that I think are significantly absent -- where's The Magnificent Seven? What about the early Tracy-Hepburn films, like Adam's Rib or Woman of the Year? How about the Clint Eastwood westerns, like The Good, the Bad and the Ugly? Or, for that matter, Breakfast at Tiffany's? (sorry, Eva!)
Some of my all-time favorites are on here -- The Graduate; Butch and Sundance; American Graffiti; Streetcar Named Desire; It Happened One Night.
It is true, regarding the older films, that they use a different visual language and different acting style than we're used to, and rely on a different frame of cultural reference -- some of them, I think, are timeless, and others just can't transcend it -- I think The Grapes of Wrath,, for example, is just as timely and moving today as when it was made. But Rebel Without a Cause -- which certainly was a significant film for its time and certainly belongs on the list -- has not aged well, and is pretty difficult to watch now.
I'm so glad Diana mentioned Citizen Kane and It's a Wonderful Life -- I usually get in trouble when I say I don't like these movies, particularly the allegedly heartwarming "Wonderful Life."
I also disliked The Third Man -- don't know how many of you have seen it, but not only is it boring and confusing, but the entire musical score consists of zither music. Oy.
I'm also going to get in trouble when I say that I never liked ET, Rocky or Star Wars, but I understand that they're significant films and I think they should be on the list, regardless of my personal taste (Just don't make me ever watch them again).
I'll get in even more trouble when I say that I thought Forrest Gump doesn't belong on the list at all -- maybe, possibly, for the technical gimmicks involved, but I doubt it. Tootsie?Another gimmicky film, whose moral was that men are even better at being women than women are. Puh-leeze!
And GWTW? Now, I'm going to get tomatoes thrown at me big time. It's historically significant as a cultural artifact, but it's chock full of racism, and its biggest "love scene" is a rape. Regardless of whether that was OK in 1939, these elements are so central to the film that I can't get past them now.
Finally, Amadeus? I enjoyed it well enough, but one of the best films of the century?? 
I'll stop now.
Helene
P.S.-- Eva -- If you're so inclined, give Breakfast at Tiffany's another try, but definitely skip "An Affair to Remember" -- is is a film that might have been OK for its time, but should be left to RIP -- but, of course, most folks don't seem to agree with me on that one, either.
"We reject as false the choice between our safety and our ideals. Our founding fathers, faced with perils we can scarcely imagine, drafted a charter to assure the rule of law and the rights of man, a charter expanded by the blood of generations. Those ideals still light the world, and we will not give them up for expedience's sake."
--President Barack Obama, 1/20/09