The X button was kind of an underwhelmer, I was hoping for something like cover amongst other things, turns out it is only good for deploying equipment, such as shield grenade (called the Bubble Grenade from here on in) and trip mines.
All images and information taken from the documentary, YouTube below,
The Assault rifle now has tighter aim, it has been brought on to replace the dual wielding SMG's as the preferred setup. Reasoning is here with dual wielding grenades and melee takes a back seat, hence the assault rifle is supposed to provide a more diverse gameplay experience by encouraging players to melee and throw grenades.
Bubble Grenade is a little ball, drop it with X and it surrounds you in a shield protecting you from incoming shots. Given it is spherical it will move around on the curved ground.
Replacing portals are 'cannons' which propel players through the map. The advantage of this (or disadvantage) over teleporting is that it gives other players from the opposing team the ability to kill opponents while they are in mid flight.
Definitely the coolest feature in Halo 3 and one that I'm sure the folks at Red VS Blue will be making much use of is the ability to freeze or go into bullet time and manipulate the camera in all manners. Snipe a guy, watch him getting sniped in slow motion, then watch him getting sniped from an action movie camera angle, etc, etc. If he's your friend rub it in, save it and send it to him, make a little movie out of it and put it on youtube, be your own movie director. No need to wait up for Peter Jackson.
Just for the love of it, some more Halo 3 screenshots. Bungie is emphasizing that all this is Alpha stuff and that the final game will look even better, heres hoping it will. Oh and Frankie, you promised the water will look absolutely stunning and have accurate physics (stuff about warthogs going through it and making waves), didn't see any water physics at all in the alpha, hope you weren't just pulling our legs.