Imho, Microsoft buying GitHub is a bad idea.

• If GitHub is owned by Microsoft, it loses its neutral role and I am very sure that companies will remove their source code from GitHub; maybe not even re-uploading it elsewhere. It might be harmful to OpenSource
• Microsoft might decide to introduce unpleasant changes for some of us, like requiring a Microsoft account to login at some point or adding certain legal terms for uploaded source code.
• Many FLOSS developers have negative feelings towards Microsoft for various reasons. They might remove their source from GitHub and re-upload to different other sites, increasing fragmentation (which also has its merits)

I'm not so worried about it. Microsoft has been much friendlier to open source in the last 5 years. They open-sourced .NET Core, VS Code, TypeScript, etc. They have a community edition of Visual Studio. They are one of the most prolific contributors on GitHub already.

They already have their own competing solution, Visual Studio Team Services that uses Git, so they're not buying tech. (I honestly think VSTS is nicer tool than GitHub. Plus they offer a free tier that has private repos unlike GH).

Some have speculated that the move is so that they can push people to Azure, which is the real moneymaker. I would think that they wouldn't want to alienate developers away from that by mucking the product they all love.