> the default designs tend to feel old-fashioned, giving a false impression that it won't do what you need

Who is actually getting this impression? What thing that they "need" is in doubt?

> I guess I assume "BS" means "UX flourishes that most end users are used to,"

You assume incorrectly. Every OS and DE finds some way to be obnoxious, even when you've learned the tricks and keyboard shortcuts. XFCE just seems to have the least of them. It's predictable. I think a new user will be able to navigate it immediately. I don't know about KDE, but I sure couldn't say the same about Gnome 3.