I'm of the opinion that the absence of complaints is not a strong indicator of good usability. I'd like to learn more about this idea, whether it's true or not, and how to argue the point if true.
My argument is that we, as humans, put up with tons of annoyances on a daily basis and we simply don't have the energy to take action on every one. Not only that, but we may not even recognize the problem (whatever that may be in life.)
Does this kind of behavior have a name? Knowing the name would be a good starting point for further reading.