I know virtually no one here will agree with me, but No Country for Old Men.
I really liked it until about 2/3 into it, then it just fell apart and made me hate it. When I saw it in the theater, people actually boo'ed when the credits came up, so I know I'm not completely alone in my opinion. It's a shame since I love almost all other Coen bros. movies.
Also, both Lost in Translation and Sideways put me to sleep.