Basically it's just...
There's so much going on in this demo that's it's really hard to pick where to begin describing it. I talked to the developers the other day, and they said the purpose of this freebie is to give you as much of the multiplayer game as they can in a reasonably sized file. So, you give Naughty Dog 1.5 GB, and they're giving you four maps, four competitive modes, two co-op modes, the much-hyped cinema mode, the ability to create custom games, access to new skins, about 80 percent of the boosters you'll find in the final game, and so much more.
http://ps3.ign.com/articles/102/1023958p1.html
That's way too much content for a Demo. Honestly, when the public demo comes out, I think people will most likely not buy the full game because basically the multiplayer content is on the demo :|. Oh well, I'm going to pre-order at Gamestop soon though, with MAG.
EDIT: Uncharted 2 also has twitter support on it wth lol.