Here is a brief list of features Authorea offers:
Multiple writers collaborate on one paper (also offered by Google docs) \cite{Riley-Huff_2010}
Automatic locking of paper sections (not offered by Google docs) http://productforums.google.com/forum/#!topic/docs/HBB8p5ke9lQ
Search and cite academic sources within Authorea (may not be that important for us but also not offered by Google docs) \cite{Zegeye_Tegegn_2008}
Add citations from non-academic sources by adding manually to citation system (not offered by Google docs) \cite{MOI_Aitera_2012}
Add images by drag/drop (also offered by Google docs) (see below)
Comments per individual paper section (not offered by Google docs)
Export to PDF or Latex format
Ability for writers to see how the entire paper reads and writing content to fit the theme or style of other writers (not exactly a feature but a good bi-product of features)