I've noticed that sometimes I will watch an older movie (such as Apocalypse Now) and since it has been imitated so many times it appears to be contrived. Basically, so many other movies have "borrowed" from it that it almost becomes a caricature of a genre, rather than a movie that helped launch a genre.
Take a movie like The Matrix. I remember being blown away by that in the theaters; particularly the fight scenes. I recently watched it again and those same fight scenes seemed tired and overdone since I have seen those same effects a million times over now.
I'm kind of a movie nut so I try to catch up on a lot of the movies I have missed, but the big one I have never seen is Titanic. I've also yet to watch Raging Bull. I saw Taxi Driver once, but was stoned out of my gourd and remember very little so I better catch that one again.