World of Warcraft does a great many things right and exp is one of them. Rested EXP bonus is much more favorable than the current fatigue system. Even the in-game messaging is silly. "Your skill bonus increases." Yaaa... sure it does. Just tell me the truth and say I'm being penalized for playing to much. How people react to systems is all in how you present them.

Bottom line - the fatigue system is presented and feels like a punishment.