First thing I should mention is that, when it comes to film, I'm easily amused. This may seem ironic, considering how much of a snob I am about books. However, it's really a matter of how much I have invested. A book takes time; especially the ones I read. So if I'm going to spend that much time and cognition on something, it had better be good : as in, both entertaining and nutritious.
Movies don't really expect that much from you. You're pretty much the passive observer. Granted, some films are meatier that others; if it's a really good film, you'll find yourself watching it again or maybe even discussing it with friends. But for the most part, movies rarely expect more than a couple hours of your time. I'm personally of the opinion that few movies deserve more than 90 minutes, and my level of expectation increases in proportion to the movie's length.
Having said that, I think most movies are pretty okay. They're good enough to hold my attention, but not good enough to be particularly memorable. I don't come out of the experience feeling 2 hours closer to my inevitable demise. It's more of a rational cost/benefit analysis : I realize that I had a couple hours to burn, and I spent that time being vaguely entertained. Now I can go on and do something else -- maybe grab a snack. I would say about 80% of movies fall into this category.
And then there's the movies that are bad. And I mean REALLY bad. I don't mean movies like The Room that are so bad they're good. I'm talking about movies that require the most extreme effort not to fall asleep or walk away entirely. I'm talking about movies I cannot say a single good thing about. Movies that fail, but not in any hilarious or interesting way -- the cinematic equivalent of an infomercial for a product that never existed. It's hard for me to say what exactly it is that makes me feel this way about a film. It's sort of like the Supreme Court's definition of pornography : I know it when I see it. Some notable examples : Magnolia, Armageddon, Chasing Amy, Forrest Gump, Seven Pounds, Primer, Crash, and Look Who's Talking. These are films that nobody should ever watch. I'd say about 10% of all movies fit into this category.
And finally, we've got The Good : movies that I would actually recommend to somebody else. I take these things very seriously; if I recommend a movie to you, I will stand behind it with every fiber of my being. These are films that I would be willing to watch more than once -- something that I rarely do. Some examples : Network, The Maltese Falcon, Belle de Jour, Fantastic Planet, War Games, Brazil, The Conversation, Dawn of the Dead, and Midnight Cowboy. I'd say that about 10% of all movies are good enough to recommend to someone else.
So there you have it : my 10-10-80 rule of film. I think it adequately sums up my opinion of the cinematic arts.