One of the big problems of MMOs that people like you and I suffer, is that we have alt-itis. After the initial fervor of an MMO the population becomes weighted heavily to the level-cap end. The lower-level end of the game being much lower in population and spread across a larger number of areas in the game, diluting the player density.
People who level up after the main surge have a different experience than people who participated in the initial rush.
The problem gets even worse later in the game when the majority of people are raiding and not leveling. When a guild is raiding, there are often needs for specific specs/classes for certain content. Guilds can either recruit that class/spec or they can have someone level an alt. Levelling that alt, on a server that has a low player-density is mostly a soloing affair. Few new people are met, few groups form, much content is simply not able to be seen.
It's even worse for people joining a game late. At least people leveling up new characters for their guild have the provenance of their own level-capped character and guild to rely on for support.
If you join the game late everything looks like a wasteland. Good luck leveling up a character that can't solo. It paints a far different picture of an MMO for the people that start late. I am arguing that this keeps people from joining new games where the leveling isn't trivial (maybe this is why most modern MMOs have trivial levelling). There is a critical-mass of players that makes things like group quests, PvP battlegrounds, and dungeons possible. In the middle-age of most games, there simply isn't the density.
I'd like to define a term: level density. Level density is the number of players in a certain level range compared to total players of all levels. For a mature game like WoW, the level density in the level that represents the level cap is a significant proportion of the number of players of all levels. That's a feature of mature games with level caps. All the players eventually move to the level cap and only new players joining or old players rolling alts contribute to the level density in the level 1 - (level cap -1) range.
Finally getting to the point. My idea is two-fold. First, you can only earn X% of a level per day. Secondly, all experience over this X% experience max per day goes into a pool. Those points are divided up at the end of each day such that lower-level characters get a larger proportion of the experience than higher levels do. This might have to be done in level brackets to make leveling in the highest levels below the cap more fair.
My hope would be that the net effect would be clumping up players in levels, making the level density graph clumpy. This would mean a group of players that started together would be less likely to drift apart in levels as they level up. I'm sure it would be unsatisfying for the higher levels in a given group.
The way the idea came to me was imagining a raft-race. The people on each team can move around on the raft all they want. At any given time some will be closer to the finish line than others but the group moves with roughly the same speed.
I think it would help people level up with a group of other people, slowing down the power levellers and boosting the ultra-casual.
I'm not sure the idea would go over very well with highly dedicated players, the type who normally rush a character to the level cap as fast as possible, and frequently do it multiple times. Maybe that doesn't matter, I suppose it depends on how much screaming on forums a developer can tolerate. Certainly it would effect a minority of the player base.
I'm also not sure it's terribly fair to give slower leveling players a free ride on the backs of the faster leveling players. I'd likely be a beneficiary, but I don't think I'd want to be. Essentially my accomplishment of leveling (small enough accomplishment as it is) would cease to be mine, and become the result of someone else's extended gaming. I do agree that it's an issue though, playing alts in Rift right now makes that very clear, as Silverwood, which was a bustling hub of massive events at the start of the game, is now a sleepy woodland zone that rarely spawns major events. I have a level 17 and a level 12 that have (between them) never earned a blue shard and never even seen a zone invasion. Compare that to my first two characters, who had multiple blue and purple shards, bought all the planar gear they wanted, and then moved on to the next zones dripping in superior and epic gear. My alts are getting an entirely different gameplay experience, and it's a distinctly inferior one to my original characters. If I were to start playing Rift now, with rifts and invasions having such a minor impact I would probably write Rift off as a yet another WoW clone and move on.
I'm not sure what the solution is though. Sooner or later everyone will level out of any given range except the newbies and alts. The only real way around it that I can see is to make content independent of level density, but doing so results in a solo quest grind like modern day WoW. I don't think that's a real solution either. If I did, I'd still be playing WoW.