A lot of "classics," simply because I doubt I'll like them.
For example, I don't like Hitchcock movies. I know they have a really important meaning in the history of cinema (especially horror movies and thrillers), but...I base my opinion of films largely on how much of the time my emotions are successfully manipulated. So in other words, if I'm not having fun during at least 60% of a "fun" movie or feeling the power of a more serious movie for a majority of the time, then it's hard for me to like it. To me, Hitchcock is all about an hour and a half of largely boring buildup to one or two good, thrilling scenes.
Others:
-Other really old "classics," like Citizen Kane or Casablanca. Everytime I try watching some old black and white classic, I get seriously bored. I think it's hard for me to relate to the characters because of the huge generation gap, and it's hard for any of it to effect me when the resolution is so low and the sound quality so fizzled. Also, all those "classic lines" have been repeated so many times by now that they're cliche and annoying. Might not have been at the time, but it's hard for me to not get annoyed by stuff like that. Mind you, I like it when modern films use black and white like Sleepy Hollow, Sin City (though I disliked that movie overall), etc. but OLD black and white movies, well, I usually don't like them. The only ones I remember liking were To Kill a Mockingbird and Inherit the Wind, because they were both about subject matter I felt strongly about.
-I just don't like Stanley Kubric movies, so I don't give many a chance. Watched 2001 and was bored stiff the entire time. I'm afraid to see A Clockwork Orange because I'm afraid it'll disturb the living s**t out of me.
-I watched part of The Godfather: Part One, but wasn't gripped enough to watch the rest. I'm not into crime films, I guess. I liked the first two hours of Apocalypse Now, though I hated the last half hour on the acount of its super-slow pace.
-Interview With a Vampire. I watched the first half-hour, got disguisted and turned it off, but I feel a little bad about that because it's one of the most well-regarded vampire movies ever and, as someone who writes about vampires, I feel like I have a little responsibility to know this sort of thing. (Maybe I'll read the book sometime.)
-I finally got around to watching Seven Samurai recently, and frankly I was bored to tears throughout most of it. I can respect the amount of work that went into that, and normally I love long epic movies, but...it just didn't get to me. I'm sorry.
-All kinds of 80's movies, like The Breakfast Club. Sorry, but I hate 80's movies. I just hate the mindset, the music, the style, the low resolution, the low video quality, the attitude, the fashion, the hair, the lack of any decent visuals, etc. It just seriously turns me off. I didn't even like Ferris Bueller's Day Off, when practically everyone likes that movie. I thought it had a few very funny scenes but not enough for 2 hours, with too much boring fluff and fat, and I didn't like how cruel Ferris was to his best friend. But I'm not a hedonist at all--I'm not the sort of person who'd have any fun with ANY of that stuff that they did the entire day. I'm the sort of person that'd just have more fun in school. So it's no wonder I didn't like it.
-Rocky. I hate boxing. It, to me, represents much of what is wrong with humanity. Wow, let's go watch people pummel the s**t out of each other! No thank you. I know that Rocky is about hope and all that, and that boxing takes a backstage, but I doubt I'd enjoy watching this movie.
-Kill Bill Vol. 2. I watched the first one and absolutely HATED it because it was so friggin' gory. Everyone else is like, "Oh, it's so funny!" Yeah, whatever--that tone was 100% serious and depressing most of the time. I never got around to watching the second one because I hated the first one so much, even though I heard it wasn't as violent.
-Other Quenten Tarantino movies. I know Pulp Fiction is supposed to be a classic, and I might like the storytelling style, but the violence sounds way too much for me.
-Gandhi, but I've got it on the Netflix Queue.