Content Designer William Fairfield has written an in-depth blog entry that explains how the ArenaNet team is designing dungeons in Guild Wars 2. He introduces the members of the dungeon team, outlines how a dungeon starts off as a paper design and goes through internal playtesting, and even gives some details on the Ascalonian Catacombs and Sorrow's Embrace dungeons.

Guild Wars 2 doesn't have the "holy trinity" of tank, DPS and healer for dungeons. Instead, it's all about the team as a whole. Here are more details from Fairfield:

"Traditional MMO boss fights consist of one character running up to get aggro, another character healing them, and the rest of the team just slamming their heads against the boss. Instead, we have players making conscious decisions in combat about where to stand, what role to play, when to roll out, etc. Overall, our dungeons have a more team-oriented feel to them. One person does not dictate the flow of a dungeon; the entire team contributes to the direction. This could mean making decisions as a team on which path to take or splitting up to fight two pieces of a boss at the same time."

For more details on dungeons, check out Lore and Continuity Designer Jeff Grubb's recent dev blog entry that covers the entire dungeon system.