This is from the resent game informer article, leaked a week early, lots of info in the article, I'll post a link a bit later.
The first Elite shown looks awesome, the second one looks, well odd.
I have to say, I am feeling more and more iffy on Halo 4 with every new piece of info that comes out about it, it all looks strange as hell to me.
Looks like the first mission you are on the Forward Unto Dawn, you are awoken by Cortana and informed that the ship has hostile intruders aboard, they are Covenant, Cheif makes a comment after fighting a roaring Elite in an elevator shaft that it is odd because the Covenant and Humans are supposed to have stopped fighting, he then looks outside to see that their ship is floating through a large Covenant fleet, near the Forunner world.
Spartan Ops sounds cool, you fight in an ongoing campaign, each week a new Cut scene and four new missions for you and three other players to play together against the AI, it is the new Firefight.
Also it has been said that the new primary enemies in the series are much different from the Covenant they are more organized, dangrenous and not just new skins, the first mission is classic Halo to get people back into it, after that they start to mix things up, new enemies and the like, it is going to be the largest sandbox change in Halo history.
I love the Covenant, they are a huge reason why I love Halo, so for me I am very happy that they are returning, and also happy that we have new stuff to fight as well.