Eagle eyed gamer out there may notice the little bit of text on the starting graphic when you boot up Diablo informing you that it has begun downloading data for a "future patch". This download is in preparation for the March 25 launch of Reaper of Souls as well as a pre-expansion patch that will be released in the coming months.
The download will be around 6GB in size and include things like models, textures, art files, and other upcoming content in patch 2.0.1 and Reaper of Souls that is already complete.