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.
City of Heroes, apparently, doesn't have any loot -- character advancement has to do with more linear ability improvements that are easier to scale up and down. WoW, of course, has gear and loot and buffs and stats to confuzzle even the best players of more straightforward games. It's not as simple as "better" or "worse." The fact that less-experienced players so often find themselves all geared up in all the wrong ways demonstrates how very complex the system is -- at some point every WoW player needs to sit down and do the math or the research on his gear to decide which armor benefits him most at whatever task he wants to accomplish in the game. If reasonably intelligent people have to work to get the hang of this, then getting the computer to understand and modify these choices upwards or downwards seems doubly impossible. (The closest thing we have to this now is the "find an upgrade" function on the armory. Certainly this could work well in some characters, but for many others it would get things totally wrong. It's fine if a website gives a bunch of strange suggestions, but if the game mechanics mess up the stats it could break things on a whole new level.)
So, it looks like a mentoring system couldn't work out for us in WoW because of this gear complexity, but some posters on the Elitist Jerks forums found other ways to work around it. A few posters found that they could remove or change certain parts of one's gear for example, and with some classes, use lower spell rankings in order to simulate a lower level with a high-level character. For me that sounds like too much hassle to be fun, but it's certainly one way to approach the issue.
In the end I fear that, unless friends happen to have alts at around the same level (which actually isn't all that unlikely for some of the more prolific altaholics out there), they'll be spending most of their new leveling time by themselves or with strangers. Of course some of these strangers might become new friends, and that's always nice. But for old friends who want to keep overcoming challenges together, the only way will be to just switch back to their mains and let their alts get some rest.