Lockouts. Nearly every major fight in nearly every quest has barriers that spring up preventing the whole party from entering the room together. The first person enters the room, the barrier springs up, and the rest of the party have to click around trying to find the little swirly icon to get in. This usually results in the boss (or mini-boss) being able to pick off the party one by one.
I can't think of ONE good reason for this. If you're going to lockout the fight room, at least let the party get in the room first.