Alright, I've always been a Goldeneye 007 freak (that goes for both the movie and the video game), and right now I've become obsessed with the game anew.
You know why?
Because I've found some very secret details in the game; the most interesting one would be the fact that it exists a completely "new" Arena on Multiplayer: Citadel.
You see no signs of it in the game, but it IS there, and it's even got its own musical theme!

I think this is a great way to increase the replay value: by hiding lots of really secret, unoffical details in the games, details that can only be accessed by using Gamesharks or similar decoding devices.
You shouldn't hear anything of it in the real game, but the additions should be extremely hidden so that you can only find out about them on the Internet somehow.

I'd like more games to hide cookies like this in their memory, because this way you'll suddenly find out about them and just feel the gameplay revive again.