I usually don't like IGN, but they did a good rewind theater on the "Scanned" trailer.
They talk a lot about the fiction and how it relates to the trailer. A few "Fall of Reach" spoilers are in this so if you haven't read that and don't want it spoiled, I'd avoid this but if you had no idea what the trailer was about, check this video out. I learned a thing or two. Nothing spoiler-ish towards Halo 4 unless you don't know about the (Weapon and Enemy) Scattershot and Promethean Knights
Oh and whatever you do... don't read the comments. Spoiler central I presume.