All we know so far is they're called Criterion dungeons, there are 3 mentioned so far, and the difficulty scales with the number of players.

I remember back in EQII where the adventure pack for the Splitpaw Saga had instances that would do this. It was quite enjoyable. Hopefully there's a story that goes along with it. And I'm curious what the rewards will be. I hope they don't punish anyone who goes in solo and just make the instance give the same regardless of number of players.