It's odd but one of the main reasons I got addicted to Minecraft so quickly was the curiously satisfying plopping sound that stone makes when being mined (with the sound of mined sand or dirt being totally unsatisfactory, always urging me back to solid rock again). I felt a bit like I imagine commander Riker must have felt when steering through these rings in that mind game from Risa he plays in that one episode of TNG.
So, apart from how great I think the game already is in its current state, the one thing I miss most (even when there are other things that I would like to be included) are those plopping sounds.