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