I'm a little confused about why people are worried. I already have the game. Many different versions of it, actually, through MultiMC. I don't even play the latest version, since all the mods I love are on 1.6.4 or 1.7.2. When Mojang has released new versions, most of the time they haven't been anything spectacularly new and different anyway, and they don't come out very often. This isn't an MMO where they can shut down the game world, here. It's largely a single-player game. I don't imagine they're going to shut down any multiplayer servers, since they're not going to want to lose customers. I think Microsoft has been around long enough to know better than to piss off the entire consumer base for a product. Even if they did something inexcusable to the multiplayer, there's always LAN play, and you can bet people will still find ways to run multiplayer servers for older versions even if MS tries to stop them, which I can't imagine they will, anyway. If they release new versions that are worse than the old ones, then... play the old ones? That's what I'm doing already. They might release DLC? Um... Don't buy it? That one was easy.
They're not going to break into our homes and take away our copies of the game. The only issue I might foresee is that future versions might have unexpected problems. Future versions might also be much better than we're used to getting. Let's be honest, here. Most of the best content for Minecraft comes from the modding community. Mojang made a great framework, but the vanilla game feels like playing with only that old big bucket of legos, all squares and rectangles. It's the mods that add all the fancy new pieces, the Technic sets, the themed sets, the spaceships and pirate ships and airships and so on.
I agree with Reelya: this sounds more like an investment than a takeover. They legally own the company now. That doesn't mean they're going to screw it up. They might not touch it at all. If they do, then just keep playing the versions we have now.