Master Chief awakens from cryosleep to find the human-Covenant ceasefire apparently broken, with the wrecked Forward Unto Dawn under siege by Elites, Jackals and Grunts. Struggle through to the vessel's hull, and you'll have the opportunity to gawp at an entire alien fleet.
343 says we can expect a new, more complex and intelligent enemy faction - founded, like the Covenant, on relations between unit types that players can exploit, but considerably more testing. You may even end up sticking it to the Forerunners themselves. After all, they've waged war on humanity before.
[Frank O'Connor, on Cortana:] "And what it really means to have your only friend be more human than you, but be an artificial intelligence. And what it means to your place in the universe to be put into a position where you have to deal with her Rampancy. You literally rescue her in Halo 3, but her jeopardy was geographical rather than existential."
