We had a chance to see some campaign at E3 and there was plenty of Spartan Ops talk on the Bombcast, but we all know the real reason Halo is still around today is because of the competitive multiplayer - now dubbed War Games. MLG uploaded the first in-game footage featuring two maps, Haven and Adrift. I've added GameSpot's commentary with the devs below for brief overview, or check out MLG's full gameplay coverage.
Keep in mind that the main thing on display is the new Infinity Slayer, hence the point scoring and ordnance drops (killstreaks) popping up. Like with the campaign demo, I hope they're just trying to show off Halo with a fresh coat of paint rather than, say, merely pandering to other shooter audiences. 343 would be foolish not to include normal Slayer and other classic gametypes in the final game.
Reach's design oversights and poor map selection made it a disaster competitive-wise, so I've been cautious with my enthusiasm in terms of how Halo 4 fine-tunes the sandbox, seeing as how there's still a strong focus on armor abilities among other significant changes. Maybe they can strike a decent balance this time around, but I picture most of the abilities (like wallhacking) being geared toward the more social playlists and left out of competitive play.