For me, it would probably be Michael Haneke's Amour (2012). I've wanted to watch the movie for like 5 years now, and finally got around to it last night.
I found Haneke'style just too sterile and distanced from its events to be truly effective. Literally everything I'd heard about the movie told me I would love it, but it just didn't work for me at all.
What are some films that you thought you'd love, but ended up hating?