Personally I'm not expecting the "performance" to be anything else than having a demo available for play. And I don't think they'd tell us any NEW news at all, maybe a quick update about runes but that's it. I've got a tiny bit of hope for a positive surprise, though.
Description: After a dance macabre from the demonic hordes, Jay Wilson and Josh Mosqueira will be on the Blizzard booth for an interview about the upcoming title Diablo III.
I still dont get why they would let us play the blizzcon 2010 demo at GC2011, 'cause for instance the skill system has been completely revised, as well as the rune system. Why let us try out something that isn't even gonna make it into the game at final release???
Have we forgotten that the actual Diablo 3 game announcement was made in Paris France at the WWI in 2009? So it could be possible we will hear something big. However Jay Wilson did a Q & A at the last Gamescom. The only major thing then was news of the artisans. Unless they talk about endgame content, which wouldn't be a big spoiler. I can't imagine any major topics bigger then the AH. Any other major announcement would be a beta date. Followed by a game release date via sometime around Blizzcon.
As a side note, any rumors you may have heard about a gate to Diablo’s Burning Hells opening in Cologne are completely unfounded. Seriously, nothing is happening at Rudolfplatz from the evening of August 16 — especially nothing to do with demonic invasions.