They have to show this game off at GDC this year. They probably, once again, won't show anything from the campaign, and instead show the multiplayer to safeguard their crappy story.
Well if you want to play as an alien so that Bungie can flesh out the alien side of their story by turning the game into a political debate, power to you, but I much prefered the straightforward "destroy Halo" plot of the original.