Stability
I am greatly impressed by the resilience of the Earth's ecosystem. If you stop and think about it, most atmospheric and other variables have stayed within an extremely tight range (as planets go) for hundreds of millions of years, despite massive continental movements, changes in the shapes of the oceans and currents, and disasters like planetary meteor strikes.
Earth has never frozen solid, nor "run away" thermally like Venus.
In fact, it's almost too resilient and stable to believe. One can easily see how the Gaea Hypothesis could almost become a religion.