Hmm. I like Minecraft the way it is right now, but if I had to add something, that would be more variety while mining. I hate seeing just the same stone blocks everywhere when I go mining, there should be different types of stone, they alreay have sandstone near the surface, but also granite, marble, limestone, basalt, shale etc. Each of these have different depths at which they are common, will have changes in the materials they carry (so that they also double as an ore detector, if you want iron, you go look for the rock type that will carry more iron.)
Oh, I guess just better geology in general, with different cave types, plains for building, then mountains for mining, all sorts of fun stuff should be added to the world. The construction is fine, I love that part, but we need more dynamics in the world itself.