I love energy. Of the three different resources in WoW at level 60, energy was definitely the best around. While death knights may argue that runes are more fun, they suffer from up to 10-second lockouts on some abilities once a particular rune is used up. Some hunters in the Cataclysm beta have claimed that focus is the best thing to happen to their class since pets. That's fine, but energy doesn't require us to play the regeneration minigame to ensure that we're producing enough.
Energy is simple to model and even simpler to use; no ability is more than 6 seconds away. The system has seen constant upgrades since its humble origins, though its core functionality has remained unaltered. It's become a crucial part of our mindset, and it defines the experience of playing a rogue. I had an old friend that used to say that all true rogues set their watches to only tick every 2 seconds. What is about energy that keeps it marching on so reliably?