Especially considering the new and improved dungeon loot, many players will wish their friends could join them. Of course, their friends could just bring over their level 70s and rush them through, but for a certain kind of player, this is less interesting because it takes away all the real challenge and teamwork of the instance. They may be able to find PUGs at that level, but it likely won't be the same.
Douglas at the Elitist Jerks forums has been having this problem for a long time now. He and his friends very much want to play together, but have never been able to make their schedules work out. Before long, their characters inevitably level at different speeds and can no longer level up together. He says he longs for a "mentoring system" like City of Heroes has, where players of different levels can become one another's "sidekicks" and go to dungeons together as if they were at the same level. At first glance it seems like WoW could implement such a system too, to make something like a temporary downgrade or upgrade in ability power and gear quality so that friends could fight together across the level gap. But further discussion reveals some serious problems.