I have been fortunate enough to be involved with the design of some of our most challenging regions. I can tell you that we emphasize having encounters/events of varying size that people would want to participate in multiple times not just for loot but also for entertainment. I can also tell you that those sizes range from individual or small group play up to larger raid type encounters. I personally tend to enjoy a semi open ended setup for progression in a raid area that lets you pick and choose. I think that Blizzard definately took a step in the right direction with the introduction of the Zul'Gurub instance as it allows smaller groups to complete things on their time while having an end boss that required completing all of the other bosses to be defeated but in no set order.

Forums, 6th December 2005 (Athelan)

One of our "mid sized" raids includes the sieging of a castle of evil mages. Along with multiple types of boss fights we want some large formation battles happening in the courtyards and the intentional ability for archers to climb up onto the walls and rain death from above. (Or push a gargoyle over and watch it fall to the ground smashing some poor saps head) Of course those evil mages probably wont make being up there any easier.

Forums, 7th December 2005 (Athelan)

You would be seriously hard pressed to make any worthwhile large group content that only took 30 minutes. I think the "saving" of a dungeon is important and I think that having it possible to accomplish something in 1-2 hours is a good target. (Thats usually enough time for even a new group to feel like they made some progress in my experience) Im not saying that to be anti-casual gamer. I just don't want to set unrealistic expectations out there.

Forums, 12th December 2005 (Athelan)

In the first phase, players might want to use quest weapons to do as much damage as possible. The second phase might occur when the monster casts an ice shield that reduces normal damage by 80%; that's when the weapons that do fire damage come into effect and those will all be crafted weapons. The third phase might require something totally different, perhaps a formation that only a formation leader can issue.

Gamespy, 25th April 2006 (Gaute Godager)