All the games are discounted, as are a bunch of humongous packs which similarly include all the games, and, if that weren't enough, Steam is giving away free games to folks who play other games. Congratulations! We all get more games than any of us could ever hope to play in our lifetimes.
Those free games are earned by completing a series of in-game objectives which cycle everyday until January 1. Completing them either gets you a freebie off the "Great Gift Pile" or a lump of coal, seven of which can also be traded in for an actual gift. Alternatively, you could save your lumps up -- each counts as an entry into a Steam sweepstakes, the grand prize for which is every game on Steam ever. Seriously, that's not hyperbole. The prize is every game.