Well, right off the bat, I notice that the "Loading..." screen pictures are now greyscaled. The "Targeting mode" is now called "Hardcore" - and you die really easily. Not sure if anything else is new, nor do I know the difference between "Expert" targeting or "Hardcore" targeting. Does anybody know any other differences?
One complaint I have about the GotY Edition is that if you want to play Multiplayer, you need to put in the second disc, which contains Undead Nightmare and Multiplayer.