Hyperskeptic, I am really excited by this thread! I also noticed Eponine's recent use of the term Relationship Anarchy and it really resonated with me.
I think you are right--it overlaps with poly in many ways but also differs from poly.
The biggest difference between polyamory and RA is that poly is "love-based." Specifically, romantic-love-based. I have always struggled around the label* "poly" and the idea of a "love-based" approach to relationships. Because while love is nice, it's not everything to me and it's not what I seek first.
In fact, polyamory feels restrictive to me because of the expectation that I ought to be seeking/developing romantic love. I have always thought, "But that's why I don't want to be monogamous--because of the expectation that romantic love is the ultimate aim. So can I really say that I'm poly?" Polyamory doesn't go far enough [away from the norm] for me. In that sense, I think that yes, RA could be viewed as more radical than poly.
*And labels are important to me for the pragmatic reason that they are VERY useful for online dating. I need various labels to describe myself in my profile and to search for like-minded people. Maybe I won't care so much about "What am I?" when I'm more settled in whatever relationships will work for me, but I am very much still searching--for myself and for those who are compatible with me.
Single, straight, female, solo, non-monogamous.