What's your opinion on unlockable content in video games (music, cutscenes, etc.)?
I would love it if ALL games had unlockable music to listen to. A few of my favorite games have unlockable music: Smash Bros., Donkey Kong Country Returns, Cave Story, Kirby's Return to Dreamland, Kirby's Adventure, Sonic Generations. It's a great reward for beating a game.
Another thing I would like to see more games have is unlockable cutscenes. Metroid Other M did it right, by combining all the cutscenes into an actual movie, with bits of gameplay strining them together.