Juristo, N., Moreno, A.M. & Sanchez-Segura, M. (2007). Analysing the impact of usability on software design. The Journal of Systems and Software. 80. 1506-1516.

some basic stuff for the background section on wtf do I care about design/usability in the context of community interaction.

on interface usability and functionality:

As a result of this analysis we are able to demonstrate that usability really is not confined to the system interface and can affect the system’s core functionality.

on usability (interaction >> interface):

Usability deals with the whole user–system interaction, not just the user interface. The user interface is the visible part of the system (buttons, pull-down menus, check-boxes, background colour, etc.). Interaction is a wider concept. Interaction is the coordination of information exchange between the user and the system.