During Bungie's GDC panel earlier today, which was all about world-building, was this bucketload full of concept art, most of it unseen until today.
Update - Here's some video from the event as well, an animation showing off how characters went from the drawing board to the game.
The images you'll see cover all aspects of the game's pre-production, from designing worlds to characters to enemies to individual locations.
Some of it has been cut, some of it will look different when the game is actually released. But all of it looks absolutely fantastic.
To see the images at full-size, click the "expand" icon in the bottom-right of each one.