Is the demo the one we saw in the Gamescom preview with high level characters or is it the "Starter Edition" from the game start to skeleton king? I know that the Gamescom demo probably lets people play around more with the abilities/combat but I think the "Starter Edition" is better into the learning curve and gets you addicted to loot gathering so you want to upgrade to the full version.
I hope the expansion comes out in very early 2014 for PC and Console almost simultaneously and I do hope it is DLC for the 360. I know Blizzard has not confirmed they are even working on it, but now that the foundation is there for the console, I would hope they are thinking port the entire time so it will be ready. A new act, new class, new level cap, new loot and a new artisan (mystic-transmog).
PVP, aka Brawling, is junk in the game. It is basically just a free for all 1v1v1v1 area you can enter with your party and have at it. No teams, no scores, no leaderboards like you see in an actual "arena" type PVP setting. In this game, PVP comes down to gear and who has the best. It can be an interesting side for a few minutes with friends, but if you are reaching outside your circle of friends to PVP, then expect to meet a lot of dupers.