Early reports from some show attendees:
The PvM demo is virtually identical to the Diablo 3 beta build. Characters start off at level 1, just outside of Tristram, etc. We don’t know yet if it’s been updated in any ways, such as with the new Demon Hunter skills layout.
The PvP demo is brand new and huge. New green-tinted, swampy tomb-style tileset. Grug said he thought it might be from late Act One. The Arena is deathmatch style, 15 minutes in length, with total kills by team for the win. Matches feature Clvl 60 characters with around 9000 hit points. The matches are 4v4, and are super intense. Grug said his team lost his first match 49-50 on the very last kill.
Hopefully we’ll get some footage of this in the new gameplay movie, since it doesn’t sound like we’ll get much new in the PvM gameplay movie, at least if Blizzard follows their past practice of only showing scenes from the playable demo in the Blizzcon gameplay footage. Screenshots and the gameplay movie should hit in a couple of hours; Blizzard doesn’t release the press kit until after the reveals in the opening ceremonies.