The 3 major things Notch and Jeb need to get working on:
1. Weather Transitions, when the weather starts clouds should roll in, and it shouldn't immedietly begin raining. Plus it could also be comprehended as the transition between rain and snow, it is decent but looks a little weird.
2. Furniture, basic structures that can be used to make more elaborate furniture pieces, would allow those big homes you have to actually be filled up.
3. Nether Update, they need to make it more in-depth and like the real world, with more then 2 creatures and a bunch of different "biomes", as in I don't want to see the same style cliff or cave, I want to see some crazy ass stuff in that crazy ass world.