The fatigue system in Allods Online is pretty smart. Players have a fatigue pool that is filled every time they gain experience. Fatigue is gained on a 1 to 1 basis to experience and is displayed as the blue portion of the experience bar. At any time, fatigue can be turned in at an innkeeper for experience (also on a 1:1 basis). The fatigue pool is limited and only so much can be gained in a single day.
This is not immediately obvious, but when fatigue is maxed out, a player is effectively cut to 50% of their normal experience gain. However, Allods does this in a very smart way which I agree wholeheartedly with.
Experience that is gained off kills and quests is never affected by fatigue. If a monster is worth 10 XP, it is worth that same 10 come full fatigue. So, the player never sees the decrease in their active XP gains, which show on-screen after a kill or quest turn in. The loss comes in the lack of fatigue gains, which are buried in the combat log. This is a smart visual way to prevent players from feeling shortchanged.
Secondly, the system encourages players to do other things. Each day the fatigue system sets a soft cap on how much experience gaining a player should be doing. Players that push past that soft cap are never prevented from doing so. Players that hit the soft cap and want to stop, don't feel punished for doing other things because realistically they are only missing out on 50% XP gain. This allows for and encourages social behavior.
Also, it encourages players to log out, which for a free-to-play game like Allods is golden. Logged in players cost money and without monthly subscriptions, its money lost. I suspect that players who are actively engaged in other aspects of the game outside of combat are more likely to drop a few dollars on a customization cash shop item as well.
Lastly, fatigue rewards the casual player. Only getting an average of thirty minutes a day to play has not made me feel left out in Allods. I can complete a few quests, get some fatigue gain, hit up an innkeeper and feel comfortable that progress was made. I often think of it as double experience every time I log in since so many of my friends are often playing well past maxed fatigue.