Something I really value in game design is inter-connected worlds and the joy of discovering the ways in which those worlds are linked. I think that's why I love games like Dark Souls and Metroid Prime.
This interesting video does a great job of explaining that joy of discovery.
It focuses a lot on Dark Souls, which makes sense. Dark Souls is one of the best examples of this kind of design. It's a real masterpiece in that regard — especially in the organic way it shows you areas of the game, the way it makes the universe feel like one real place.
Well worth watching.