Minecraft needs the following things, pronto:
* Special resources in different parts of the world - not just in different biomes, but several (in-game) days' walk away from each other. Special stones (Ruby? Emerald?), different kinds of plants, rare materials of all kinds.
This will accomplish two things primarily: First, it gives exploration a real purpose besides "see scenery." Several versions ago, I used to go on treks to find cactus (back before there were entire biomes of the stuff), reeds, and flowers. It was a great change of pace from building and fighting, and it needs to come back.
Second, different resources will give multiplayer a new dimension. Imagine if everyone started in different areas of the world and could trade their unique resources back and forth to other communities.
* Survival mode - let the player need to eat, sleep, and keep cool. Let the various attributes kick in after a few days' time - say, on day three you feel hungry for the first time, and on day four heating becomes an issue. Suddenly, the game gets an added layer of complexity, making for a much more involving experience.
* Better dungeons. The current dungeons are a nice start, but there's so much more to do with them. Persistent monsters, chains of rooms, traps... dungeon raids could be so much fun with a little effort.
That said, I love the idea of this achievement system - it's a perfect tutorial for the game, at least if it stops after a while. If it keeps going with various and sundry recorded "discoveries," though... not so good.