I can think of only two movies where the detail made a big difference, Pan's Labyrinth and Pitch Black. The Pitch Black version is an extended director's cut.
BUmping thread for interest, wanna know which other movies are especially good on Blu-Ray
know this is going to be totally controversial, but am just wondering if Blu-Ray systems use some audio channels better, for example Gandalf's voice during the initial sequence with Bilbo after the party and firecrackers in the first LOTR movie sound different when watched on HBO and on Blu-Ray using an otherwise same setup
And the first 20 minutes of Star Wars Episode III, pretty amazing in HD, waiting for blueray