What are your favorite RPG mechanics?

I'm looking to construct my own system, but I'd like some input on what the best "pieces" of each ruleset are.

As I'm very familiar with D&D 3rd Edition through 4th Edition, I'm looking more to learn about the other systems.

Please don't just mention a system in general, but the specific parts of that system that make it better than the others. I.e., maybe you wan to mention Exalted's method of turn-based combat (where each player has a speed rating, and faster characters actually get to go more often, rather than just first).