Good deal for you people who like the extra goodies.
In addition to the game disc and manual, the Limited edition includes:
Feature 1 : Game disc housed in recovered ONI "black box"
Feature 2 : An exclusive Elite armor set for use in multiplayer modes
Feature 3 : Artifact bag containing Dr. Halsey's personal journal and other classified documents and effects that unravel long-held secrets from the Halo universe