I always try to be careful when it comes to judging things I didn't like. Things don't become regarded as classics for the lulz, a classic is something that has received enormous amounts of praise over a very long time by a lot of people, both high brow critics and average Joes. and just because I don't like the game/book/movie/whatever that doesn't mean it's bad, and claiming that everyone who came before me and praised it is wrong while I'm right seems a bit narrow minded. There are a lot of classic works (mostly movies, I don't read that much :-/) that I don't like, but more often than not I can admit that I don't like it because I lack the deeper understanding for it that the people who like it have.
That said, the prime example of a movie that I didn't even remotely get is Akira. After watching it I went online to read up on all the themes and subtexts that were in that movie, and it really baffled me how I got absolutely nothing of that while watching it. To me it was just a weird movie about a kid who got superpowers and decided to wreck Tokyo for the lulz, with some creepy 90-year old babies and milk-leaking teddy bears thrown in for good measure. I should probably watch it again :-/
I also have a hard time appreciating slow, long movies. If it goes on for too long with too little happening in terms of plot, I just kind of loose interest and stop appreciating it, no matter how good it is. The Assassination of Jesse James is probably the most recent example, I had a hard time staying focused during There Will be Blood too.