It's hard to say what exactly Microsoft will do for Minecraft, but keep in mind that it's possible they may be able to provide much more for minecraft than Mojang was able to. That is part of the reason that Mojang sold out, it got bigger than they had ever anticipated. I'm sure Microsoft is smart enough to realize that messing with the minecraft community's connection to the game would be a very bad business move. Why fix what isn't broken and is bringing in tons of money?

I know every one is worred, but microsoft as already sayed that the ae big fans of minecraft, and they want to keep minecraft the way it has always been. (of coures they were taking about the x-box at the time but we hope this will remain true for the pc.)