Those of you who like to barrel through MMOs with a single character are in for a rude awakening with Final Fantasy XIV's Fatigue System: if you play with a class too long, you'll eventually earn no EXP.
Every week, the class you play with has an eight hour limit where you'll gain the normal EXP amount for your efforts. After you pass those eight hours, you will earn less and less EXP until you no longer get any. Only by playing a different class or waiting for a new week to start will you start earning EXP again.
Why is the Fatigue System in place? Game director Nobuaki Komoto likens the system to what happens in real life: you can't keep training forever without getting tired. Beta testers have been hitting the limit much more quickly than anticipated so the system is being tweaked to ease up on the penalties. Still, it is rather disappointing knowing that you're being told how to play.