Indeed, a vendor's lack of cooperation with the open source and free software community should be seen as a warning signal.
Thanks for the comments on the book. I know real-time is one item some readers will miss. I made a decision not to discuss the subject in the book based on my own involvement in the real-time in Linux community. I've detailed the rational for this choice in an article available on the book's web site if you're interested (http://www.embeddedtux.org).
I've noted the other items you mentioned and will take them into consideration for future versions.