I remember that MUDs (precursors to MMOs) often had a system of reincarnation. Once you reach a certain point, you can choose to start over. Doing so gives you various perks, and you leveled up faster. I think a few MMOs have added something like this, not sure (I've only played DDO, which has something close).
How it would work: get to 80 in WoW, choose to reincarnate, get a few minor permanent buffs and gain experience x times faster (x being current number of reincarnations) until you hit your previous level again. It allows a die-hard story fan to see the content they missed as well as the new stuff, while still providing incentive with the stat buffs for the raid guilds to do it. Each time they add a new expansion, as well as raising the level cap they also allow one more incarnation and add the associated buffs.
I think the best system I saw was in a MUD I can't remember the name of (that's long since gone) where every time you reincarnated you got to keep a very toned down version of some of your old character's abilities as 'memories of a past life'. They were not overpowered, but they were a huge boost in the beginning of a new life. Not to mention, a pure warrior with the ability to detect magic or traps is awesome. The original Mass Effect had a similar system, for a more modern example.
The alternative is level scaling, which is not ideal in most situations. If you scale the level 80 players down so they can fight with the level 30's, how do you handle their equipment? Their powers? If, say, paralyze isn't available at level 30, do you just drop it off their power bar? Do you change it to stun? Give monsters a higher save? What if the entire character is built around paralyzing, and the buffs/perks they chose don't transfer to stun? And with items, do you drop the accuracy? Armor class? Damage? The abilities? With the sheer amount of potential character builds, it'd be impossible to do it automatically.
DDO has an advantage, there. D&D is calculable, as long as they followed the rulebooks closely enough. You know that, say, a DC25 save should be a DC15, or this +5 Sword of Ominousness would be a +2 Sword of Minor Apprehension right now, or a Greater Rod of Copious Explosions should really be a Lesser Rod of Gently Expanding Hot Air.