One really impressive thing the game is doing is dynamic co-op. You can play it completely alone (no AI partner) or with drop-in drop-out co-op. The story, dialogue and events dynamically change depending on rather you're just Isaac or with a partner. Speculation on how this works with the story is Isaac's partner is a figment of his imagination, but again that's speculation.
You're basically getting two different versions of the story depending on how you play. That's really cool.