I've been trying to catch up on my movie watching. I didn't watch a ton of movies growing up, and I managed to plow through a pretty big list during college, but it wasn't until Netflix came along that I was able to fill in a lot of the gaps.
I'm not a fan of lists, but I would occasionally go through various lists, popular, critical, and other, to see what movies I need to watch. For example, let's take a look at the IMDB list--it's not perfect, but it's a pretty decent balance between critical appraisal, popular culture, and mass appeal.
Going down the list--the first movie I haven't seen...well, I think I've seen 12 Angry Men. In fact, I know I've watched at least some of it, and I think it was required watching in every single junior high class ever, but I don't remember much about it. (I do distinctly remember reading the play, so I bet that was what I am thinking of.)
Next up, I haven't seen any of the Lord of the Rings movies. I know I should, but at this point every time I think about sitting down and watching them, I just remember about how it's going to take, like, thirteen hours to get through them all, and I never pull the trigger. I'll get around to it.
The next movie I haven't seen is Inception. Again, no reason; I just never got around to it. Also, I'm pretty sure I know all the spoilers and I'm not sure if it's worth it at this point. (Also--and I know this is anathema--I'm not sold on Leo DiCaprio as being the Greatest Actor Ever. He's good. Not great.)
I have not seen the Seven Samurai, although I have seen the Magnificent Seven. I know they're different movies, but they're similar enough that I'm not in any rush.
Finally, of the top 25, I haven't seen City of God. Maybe some day when I'm drunk.
So watching the LotR trilogy would chop my gap roughly in half. (I'm actually pretty good with the next 25, except for the Chaplin movies and some of the weird foreign ones. And I've been trying to get Leon the Professional for a year now and for some reason Netflix doesn't have it, even on old DVD.)
But in the end, I'm not sure it matters. There's a lot of genres I just don't care for, and others I love, and while I think it's nice to be well-rounded culturally I'm not sure it's all that important.