Minecraft's next title update (1.5) will deliver a host of new features, among them an achievement system, a statistics log and, perhaps more importantly, weather effects. The game's creator released this video late last night showing all those features in action.
Markus "Notch" Persson says there's some tinkering to be done with the weather - "Rain and especially thunder is waaay too common in this test," he says. You can see how precipitation both transitions from rain to snow given the temperature of the area; how ice re-forms and snow accumulates, and how rain will grow flowers.
As for achievements, "Ride a pig off a cliff" is indeed one offered. Notch gave it as an example last month, evidently it carried through. While he says some weather effects occur too frequently (such as thunder), I propose that "Punch a cow in the face to make it rain," be another recognized achievement.
[thanks to Jonathan C. for the tip]