Teamwork mentality. Like it or not, the BR and the writer form a relationship. There's a generous amount of effort on the BR's end that compliments the generous effort that went into writing the manuscript. When the BR commits, then the writer must commit to that relationship and foster it. It's not two people bumping into each other randomly in an alley or aboard a cruise ship.
Candor. If the words matter in the book, then they matter coming from the BR. They'd better be direct and have meaning.
Maturity. There's a difference between a "focus group" and a beta read. In the focus group, I want to assess reactions from a specific demographic. In a beta read, I'm asking the BR to run a professional review of certain aspects of the manuscript. So, in short, someone with experience, savvy and guts is preferred.
Knowledge.
Opinions are one thing. Facts are another. If the BR is vehement about something, it better come with a reference or a footnote. It's not to say the BR should be a fount of knowledge. Rather, have the ability to acquire the knowledge and then tell me where to find it.
[This is so true. On a slight tangent - I had a diverse group read a MS that featured witchcraft and witches. One girl - who was quite religious - attacked the occult elements in the book because she personally didn't agree with them. I asked her to step back and give me her unbiased opinion re: the story and she got over it. Having said that, it was invaluable for me in that I learned how the 'mainstream population' may react to such detailed information re: witchcraft. In the end, I toned it down (etc.). ]
Detailed feedback. "I don't know how you'd fix this" or "I don't like this" are OK, but I'd like more if I can get it. If those statements were followed with "but I might suggest this ..." and "but you might try this and this because of these reasons" then that's someone who I want as a BR.
As a client, I'm seeking complete objective feedback. problems without solutions are not helpful to the process. Put more succinctly, "mo' thorough is mo' better."
All that said, I'm seeking a beta reader. My post is in this forum.