THIS, so much. One time (~1995 or so?) I bought a boxed SUSE Linux release. Pulled out the book, and practically on the 1st page, there was a copyright notice specifically allowing anyone to copy that book & pass it on. Whereas for practically every other book I'd had in my hands up to that point, copyright notices consisted of various "forbidden to do X, Y or Z" statements.
That right there convinced me I had something different, something special in my hands. Disclaimer: I had read about "open source" before, so I did know what I was looking at. But still it felt special.
Unsurprisingly: note the documentation linked to at the end of jmmv's Unix<->NT comparison above. They're ALL Amazon "buy this book" links. Over on the Linux/BSD side of the fence, such references are typically web pages, man pages, a free-download pdf or the like.
It's not the "free beer" aspect that got me over time. It's the "open", "sharing", "community project", versus "closed", "profit", "property of BigCorp Inc".