Update: EA has officially confirmed many of the known details, including the new classes, maps, weapons, and Platinum difficulty. It also teased new weapon mods and upgrades, and the addition of a new escorting objective.
BioWare is slated to detail new Mass Effect 3 downloadable content packs at San Diego Comic-Con this weekend, but at this rate of leaks it won't have anything left to reveal. Fans have sifted through the game files even more, finding more complete descriptions of the new classes, powers, and uncovering a new difficulty mode.
A series of posts in the Something Awful forums detail a new difficulty mode for those tired of Gold, called Platinum. This seems like a reasonable choice, given that many of the new classes sport pretty dangerous powers. The "N7" human classes bring abilities like a Homing Grenade (Demolisher), Devastator Mode (Destroyer), and Electric Slash (Shadow). The Slayer even has a sword. The descriptions put a lot of emphasis on these new classes having dramatically stronger armor, or faster movement speed.
Note that BioWare and EA haven't confirmed the existence of the Earth pack at all, much less these classes and powers. But the pack itself was initially discovered by a fan, and confirmed yesterday by a listing of upcoming content on the Xbox Live Marketplace. So far the details line up. The pack is due July 17, and will be free like the other ME3 multiplayer packs.