The X Button Does...

Something that is not totally new or innovative but will simply be a nice feature, something thats cool as it says in the Halo 3 Beta FAQ, 'it won't be all that surprising. Just cool.' I don't mean to say I know and as it says in the FAQ the new feature may not even be mapped to the X button (there is a new feature, no doubts there) but anyways, here is a list of things the X button could be used for.

Reload Like the Previous: Boring as it sounds, but it is possible.

Real Time Weapons Change: You know you want it.

Master Chief Helmet Put Back On: It was in the pre-rendered trailer, maybe when hit with an awesomely powerful weapon MC loses his helmet after which he has to find it and put it back on with the X button.

Call in Air Strikes: Sounds like some sort of feature a squeeler would want to see, all the same it is a possibility.

Change Fire Mode: From semi-automatic to burst fire to fully automatic, though adding such a feature would probably also mean recoil will be added as a new gameplay feature in Halo 3.

Alternative Fire: Same as the above but more like changing from bullets to grenade launchers.

Sphere Shield Grenade: From the pre-rendered trailer we could see MC used a special type of 'grenade' that created a barrier around him shielding him from on coming artillery fire (of some sort). The X button may be used specifically for that shield-type grenade.

Issuing Commands: Perhaps MC will take a radical shift in Halo 3 and start ordering troops around, how it would be used in multiplayer is many and varied... lets not go there actually.

Mute Annoying Players: Only Bungie knows how a single button can mute out the annoyance in ones life.

Covering System: This makes all the sense to me, games like Rainbow Six: Vegas prove that it can be done within the context of an FPS and a lot of people have gotten used to it like it's second nature in Gears of War, it would be a feature sorely missing in Halo 3 MP should it not be included.

Will the X button make or break Halo 3? I don't know, I doubt a single new feature will. If the X button does something from the above or does something completely different one thing is for certain, Bungie wants the X button to do for Halo 3 what vehicle jacking did for Halo 2.