Fact
Aggrend (Blizzard forums, Oct 8, under the podcast episode 3 thread): earlier beta builds based dungeon kill XP on the highest-level group member, so a level 20 grouped with a level 25 got very little; the fix bases XP on each player's own level relative to the creatures. Together with the 20% increase to base dungeon kill XP, dungeon leveling at perfect XP-per-hour play should be close to or equal to optimal questing. Dungeon XP will not match Classic Era, and chain-running dungeons to 60 is not meant to be the best way to level. Wowhead reports the same from the podcast and his follow-up.