Tobold has raised an interesting question on his blog today about transferable experience and I thought I might weigh in on the subject. To summarize, he poses the question of whether or not experience should be transferable between your alts in an MMO. You could either continue to gain xp at max level that you could spread to your other under-max level toons to speed up the leveling process or you could prolong your leveling time by syphoning xp to an alt.
I had never run into the "leveling is happening too quickly" problem myself, since the game I've played the most, WoW, didn't have that problem when I started playing it back in vanilla. It took a few hundred hours to get level 60 back then and that was drawn out a little further by quest hubs not being as complete as they are now, more xp being required per level, and mounts being a little harder to come by so travel times were greatly increased. However, for an alt lover, such as myself, this does sound like a great idea.
A point that Tobold didn't raise, but that I think is a good one, is continuing to reward xp to max level characters could encourage them to go on raids or help with quests that would otherwise have no benefit to them. All of a sudden, by help a friend, or random stranger, they can get that alt mage to cap a little faster.
I'll always be in favor of giving the player different ways to play the game. If someone wants to have a bunch of max level alts, but finds the leveling game tedious, this would be a great solution. I'm for it.