I think this is both a good and bad thing. 1) They aren't touching Original Minecraft, They are making a Second game. but they dont know the nativity and thoughts of the original developers, so they might mess it up.
It is a good thing that they bought it. There only hit game was minecraft, and they didn't have the resources to continue. The game's updates started to slow down, and people probably want to see a wave of new ideas. 1.8 is a bit shaky, so new ideas will be welcomed, not that they were doing bad. There other games flopped and need some attention so Microsoft will likely give attention to those too. Microsoft also has the power of money.