I'm having lots of fun playing Kingdom of Loathing. It's silly, but that just adds to the fun. And some of the game mechanics are surprising well designed. But as much as I like KoL, I can't play it for more than about 1 hour per day. Because every day you only get 40 adventures, and most things you want to do cost 1 adventure. You can get more adventures by eating, drinking, having special items equipped, or by clan items, but all these are limited in some way. So you end up with maximum about 100 adventures per day, and then you can't adventure any more. You can still buy and sell stuff, or do other things in your inventory, or chat with your clan or other players, but for further adventuring you need to wait for the next day.
So of course I'm wondering if this is a principle that would be possible for other games as well, for example World of Warcraft or a similar MMORPG. We don't hear very much about it any more, but back in 2005 there was a story going round that the Chinese government would impose a 3-hour limit on online games, but it seems to have been implemented in 2007. While being forced to stop to play can certainly be frustrating, the idea certainly also has it's good sides.
One advantage, and we'll have to see how important that one gets in the future, is that restricted play hours per day preempt accusations of the game being "addictive" or harmful for people playing it endlessly. Of course nobody forces you to actually eat, drink, exercise, and take a shower when you daily play time is over. But by leaving the flow, there is at least some chance that you remember the real world and do the most urgent stuff there instead of playing on until you drop dead.
But as Kingdom of Loathing shows, restricting how much you can play also has an important game design function: You prevent certain gamers from playing through your game in a few days, or from advancing much faster than everyone else, just because they played 16+ hours per day. For the game company restricted play offers the obvious advantage that people would need more days to play a game through, thus more subscription fees. Of course a simple time limit is probably not a good solution, because then players wouldn't chat any more, because it would cost them time best spent adventuring. But WoW for example has a definite "in combat" and "out of combat" status, so you could for example easily be limited to 2 hours "in combat" per day. Anything more would not give you any xp, reputation, honor points, nor loot. Which means that you could still be online for more than 2 hours, and do other stuff like chatting or playing the auction house; but questing, adventuring, raiding or PvP would be limited.
From a game design point of view, current MMORPGs have a big downside: The power and status of your character depends too much on how many hours you played, and not enough on how skillful or efficient you play. If you could only gain xp in combat for 2 hours per day, those 2 hours would really count. Efficient and skillful gameplay would be important again.
And if the game is good, players would accept that restriction. Of course not if we really added it to an existing game, like WoW. But the players of Kingdom of Loathing accept limited play per day, because it was in from the start. A MMORPG would just have to be new and different for players to accept the restriction in that game as well. Being restricted could even end up making the play time more valuable to the players, and end up being more fun than endless sessions in a classical MMORPG.