Specifically, the, erm, "Collector's Edition" includes the new Cthulhu's Angels mode, in which the Lovecraftian god sends three new playable characters -- we particularly like the sound of the "Were-Zompire" Molly -- to save the world. The mode also features new music, dialogue and abilities.
Additionally, the Collector's Edition will include director's commentary, rebalanced gameplay, a new dungeon and a new "Insanity" difficulty setting. It will apparently replace the original game on the Xbox Live Indie Games platform when it's released this spring, and current owners will be upgraded for free. A PC version will also be released at that time.