GotY edition comes with every piece dlc released for Halo 4. Maps packs, preorder skins, Fotus. All of it. I dunno if you have to download it all or if it's all on disk.
It's not clear whether or not fotus armor will be included with the game of the year version of this game or not. The ad says that it will come with "Pre-Order and LCE exclusive content." LCE could either mean Limited Console Edition, or Limited Collector's Edition. My guess is that this is referring to the Limited Collector's Edition content, which would be the Recruit Prime skin, and the emblem that comes with it. I could be wrong though, since the abbreviation could go either way.
There are some other dlc weapon skins and armor skins that come with the mcfarlane series 2 toys that will be available around the same time. I doubt that those weapon and armor skins will be in the GOTY Edition since they will be available around the same time.