mmo - MMORPG design for time-limited players

I believe that there is a significant market of players who would enjoy the exploration and interaction aspects of MMORPGs, but simply don't have the time for the endless grinding marathons which are part of the average MMORPG.

MMORPGs are all about interaction between players. But when different players have different amounts of time to invest into a game, those with less time to spend will soon lack behind their power-leveling friends and won't be able to interact with them anymore.

One way to solve this would be to limit the progress a player can achieve per day, so that it simply doesn't make sense to play more than one or two hours a day. But even the busiest casual players sometimes like to spend a whole sunday afternoon playing a video game. Just stopping them after two hours would be really frustrating. It also creates a pressure to use the daily progress limit every day, because otherwise the player would feel like wasting something. This pressure would be detrimental for casual gamers.

What else could be done to level the playing field between those players who play 40+ hours a week and those who can't play more than 10?

Edit: Thanks for all of your answers. They all were very inspiring and gave me some interesting ideas. I decided that the first thing I will do will be a rest mechanic, so I accepted the answer by Orin MacGregor. But I upvoted a lot of other answers, too. When you are interested in my first draft for the rest mechanic, you can read about it on my development blog: http://ontraindevelopment.blogspot.com/2012/10/rest-system.html

Answer

Any sort of resource generated while logged out. WoW and similar games have rested experience that helps those with less time to catch up. There could be other things generated that would help others to catch up, such as a rested currency used to buy items that can buff the player, but they must be implemented carefully to not be abused by those who don't really need it.

A more complex implementation could be temporary level boosts. If a player wants to play with a friend 10 levels ahead of them they could be boosted ~10 levels. This would require a balance of implementation, like do they temporarily get the skills associated with the skipped levels? (Later generations of Pokemon have this where stats scale up but they don't get any new skills, but that's not truly an MMORPG.) Should stats on items scale linearly? Should they earn experience toward their base level, and if so should it be scaled down to their base level? Can they earn money and items?

Expanding on your mention of limiting progress per day (which Final Fantasy 14 initially did with terrible results) is to implement it like the rested experience system. Still limit people to a reasonable amount per day, and have the leftover be rolled over into the next day (eventually capping out to curb some abuse). So while a player with time every day can get 1 million experience per day, his friend that can only play on the weekend can get 5 million on Friday (assuming resting started at monday and they don't play until friday). They don't use all 5 mil, say only 3 mil, then the rest carries over into Saturday so they can get the daily 1 million plus the leftover 2 million.