I haven't had a lot of time to play over the last month. I'm in a relatively hardcore guild. As a result, I'm way behind everyone else. Pretty much everyone else in the guild has gotten to 70. In fact, I think that I have the lowest level main character in the guild.
This experience has made me really understand the desire to power-level characters. I really want to play with my guild, to see new content with them. I like levelling, and I don't really want to skip content, but I do want to play with my friends.
Levels and progression are both boon and bane to this genre. The idea of slowly improving your character is a major attraction, and just the process of levelling up is fun. But when levels are out of sync, they are annoying because they are preventing you from playing with your friends.
I have a couple of real life friends on the server as well. One just started a month ago, so all of our levels are out of whack. To be honest, there's probably no real way we are all going to be able to play together until we all hit 70. When levels no longer matter.
In many ways, this is the central paradox of the MMO genre. Levels are the carrot, the reason for playing. They represent effort and achievement. But levels also prevent you from playing with your friends. This Penny Arcade comic sums up the dilemma perfectly.
I know different games handle it differently. WoW has the level caps and the slow-down in level speed, as well as rested experience. EvE Online has skills based on real-time, rather than in-game time. I believe that City of Heroes has a 'side-kick' system to temporarily boost a lower level character up. But I don't think any of these solutions truely solve the problem.
I have no idea what the perfect solution is. It may well be that there isn't one, that this is just a total paradox, and the best you can do is make a reasonable trade-off.