The biggest Blu-Ray surprise to me thus far wasn't a movie, it was a TV Show, and a 40 year old one at that!
The Prisoner, sourced from the film prints, looks absolutely stunning in Blu-Ray. There are bits of detail i'd never noticed before and the entire series just pops off the screen. The Avengers is getting a similar treatment, or so the rumour fairies say, and I can't wait.
Others have already mentioned 2001, Dark Knight, etc. A lot of the movies I'm looking forward to the most are the older movies being remastered because (whilst this may make me a marketing mans dream) I just want to own the movies that mean the most to me in the best and most detailed format possible. Ghostbusters for me was a bit disappointing, and the Hi-Def format shows some really bad Matte Wobble and an unusual amount of grain in some scenes. Close Encounters of the Third Kind blows me away primarily because of it's source and history, yet it still looks and sounds spectacular. 'The Good, The Bad, & The Ugly' was also a revelation, despite some evidence of scratches and pops on the film transfer, the quality of the transfer is awesome. I'd rather have a few scratches and pops than have it digitally scrubbed until it looks artificial.
The first Blu I ever owned and the one I still use to show off the format and the HDTV I have is the BBC Yellowstone Documentary. It's not a movie but it contains diverse enough lighting and settings that the benefits of HD shine.