Halo Combat Evolved: Anniversary edition will include not one, but two desirable pre-order exclusives.
The first is the Master Chief avatar outfit in all its' green Spartan glory.
The second, and arguably, the most interesting, is the 'Grunt Funeral' skull, an in-game modifier that may be turned on at some point (Most likely before a mission begins.) that causes grunts, a re-occuring enemy in the Halo series, to explode in a large blue blast, much like that of a plasma grenade, an energy weapon within the series. This can chain onto other grunts, making for large explosions that are not only dangerous to the enemy but also to the player.
Footage of the Grunt Funeral skull can be seen here.