I'm going to echo Scarf's comment about WALL-E, but for slightly different reasons. Out of all of my favourite films (of which WALL-E is definitely up there), I didn't know an awful lot about them before watching them for the first time, so their greatness was a real revelation to me when I ended up watching them. With WALL-E, because of the 2-month gap between its US and UK release (and also because I'd been following news about its production ever since it was announced), I'd read interviews with Pixar, watched the trailers multiple times and, though I didn't read the Wikipedia synopsis or anything, I felt I already had a good grasp on the story before even setting foot in the cinema. When I eventually did go to see it, I was either going to be immensely disappointed or immensely elated because of all the legwork I'd put in beforehand.
If anything, I imagine I'd fall even more head over heels in love with it had I treated it like any other film, but I would like to go back and see if that would be my real reaction.