I've beaten all seven worlds and most of the dark stages and yet there's still a locked question mark before the first world. What is this world? How do I unlock it?
If you've collected twenty bandages, you've already unlocked it. You may have gotten a message after you collected your twentieth bandage that said something like, "You have unlocked Teh Internets", "You have unlocked English", or (most recently) "You have unlocked something locked."
The locked question mark is a new zone called "Super Meat World." It's intended to be a never-ending stream of new levels for the game. An update to the game in mid-January will unlock it, along with an extremely capable level editor, and an online level sharing website. Users will be able to create new levels and upload them to be rated and played by others.
From the developer's blog: