What are the expected effects of the acquisition of GitHub by Microsoft?
Microsoft has heavily invested in their Git Virtual File System. It is unlikely then that such a key system central to Microsoft's internal development processes will be going anywhere any time soon, and GitHub is a smart dovetail with Microsoft's portfolio and strategic direction.

Microsoft has recently been moving towards more freemium services - which GitHub already is - and moving towards open source - the company was one of the top contributors to open source in 2017. Other notable examples are things like:


The Linux Subsystem for Windows
The open-sourcing of power shell 
Contributions to the Linux Kernel


This also provides a strategic opportunity to integrate Visual Studio (which already has pretty good integration with Git) more tightly.

In short, Microsoft will probably do very little to harm GitHub - it will only work to make it solvent. This acquisition may have been very much about preserving GitHub to ensure that Microsoft will be able to continue to leverage its' features - the very same features you probably use. Nonetheless, guardianship of GitHub will be a difficult balancing act, as Microsoft must be careful not to alienate developers but they may have to end the gravy train as GitHub has been bleeding money for some time. Ultimately this acquisition is likely to be a good thing for the developer community as it will help to preserve and continue the public services that GitHub provides, but at worst things will look like the migration from SourceForge to whatever the new and shiny hot young thing is.