There was some usability study back in the day that concluded that the Linux desktop setups that looked like Windows were harder to use, because they set the wrong expectations. They worked differently enough from Windows in details that it was confusing; a setup that looked different gave people the right idea that they should be learning something new and made it easier to complete the tasks in the study.
A common intuition, including mine, was the opposite (that we should theme/configure things to look like Windows). I saw lots of Linux desktop deployments that started by picking the most Windows-like theme and putting the panel at the bottom and basically going through all the settings making it like Windows. According to this one study anyway, that was not a good approach.
I can't remember where this study was from, I'm not even sure it was ever public. And not advocating swapping out your parents, if nothing else I'm sure they're used to XFCE at this point.
But I do think it was an interesting counterintuitive finding.
I expect a lot of commentary on GNOME 3 probably comes down to "not like Windows / GNOME 2"; a lot of commentary on GNOME 2 was the same. If you define "usable" as "what people are used to" then essentially all change is wrong. In some cases that definition of usable is the right one. Depends on your goals.