Google added it's own version of Microsoft's "track changes" feature a few weeks back as part of its big I/O revamp of Drive. Starting today, Google Docs can import the tracked changes from a .docx file and load them up as Suggested Edits.
Just like the Microsoft feature, Suggested Edits allows collaborators to make changes to a document, which you then accept or reject. This should just happen automatically when you import a .docx and convert it to Docs format. It's essentially the same feature, so it makes sense they'd be connected in this way. Note, you can also natively edit .docx files in Drive now, but this is a separate feature.
Suggested Edits filled in a major functionality gap for Google Docs, but most people that have need for this sort of thing still use Word. At least this way you can still work with those folks without switching over to Word all the time.