Like the tittle says In minecraft update 1.5 it will add weather ! personally I am looking forward to this Also there is rumors that the mod Pistons is going to be made part of the core game
"We’ve been working on finishing up Achievements and Statistics for 1.5, I ran into some issues with JSON, got frustrated, and added weather instead. Developing new features like that is what I enjoy doing the most, so I ended up covering a lot of ground. There’s periodic downfall (snow in snowy areas, rain in others, nothing in deserts) that make the sky darker, hides the sun, moon and stars, and makes everything wet. Ice grows back in cold regions, and snowfall covers the ground when it’s snowing. During rain/snow, there can be occasional thunderstorms with lightning. I haven’t finished the actual lightning bolt yet, but everything else is fairly done.
Once the stats are done and the weather has been tested through, we’ll release 1.5. I’m guessing this will be next week. The piston mod might get added, but it will not be in time for 1.5."