I have to call bullshit on the people who claim to have seen everything from everyone or didn't turn off a single great classic.
I'd like to see Samsong's list... a weird omission from him would be quite interesting since he knows his shit so much.
Well, when I was like 16 I remember feeling weird about turning off Braveheart because it bored me. I saw it complete the next day and it bored me again. I have seen it two times since, it's not really boring, but I guess I was expecting too much of it because of the oscars and whatnot. Of course Braveheart, not seeing it or liking it is nothing to be ashamed of unless you are a 62 year old retired professional wrestler or something.
Other than that...well, its hard because I just never turn off anything even if it's painful to watch. I know I had a hard time enduring Metropolis the first time i saw it. L'Eclisse and Persona are two movies I ADORE which I barely watched complete the first time around, but a lot of these "hard" movies had a double effect on me. 8 1/2, Ran, Persona, LÉclisse, onibaba, many many movies felt like homework the first time but at the same time it was like some spell on me. In many ways it was a repetition of the first time I saw 2001 and understood absolutely NOTHING of what was going on, yet it was impossible for me to stop watching or rewatch at any chance. trying to be analytical about it I guess it's some kind of humble attitude towards film. I don't know, it happens without you noticing it. I saw 8 1/2 at 18 or something and couldn't find one single thing to say about it, but it just never left my mind, for years. I remembered entire scenes very vividly without being able to contextualize them. The I saw it again a few years later and it just connected with me. Many of these classics are not instantly enjoyable rides.
Mostly with silent films it can be hard, but then in the last 12 months I feel I've been rediscovering or finding a way to watch silent films and actually enjoy them more than normal films. I think it's all part of an evolution you have as a viewer, you go through phases or you start getting more and more easy going about venturing into anything with an open mind.
Something that I feel happens when you actually see a lot of films is that the more films you see the more films you like. this goes against the general perception of the film geek being extremely selective about liking things but I just have experienced the opposite. Yet it becomes harder to simply and blindly LOVE a film like before, you find good things AND bad things in everything.