Here via metafandom. You make good points. I've been thinking about this subject too. Over the past few months I have beta'd for *counts* seven different people, four experienced writers whose work I love, two new writers and one for whom English was not her first language.
My main problem is that I am naturally blunt and, when there is a deadline that the writer has to meet, I don't have time to 'smooth the edges' of some of my comments, as I know I should (to use my best friend's expression, who is also on the receiving end, all to often).
As a result, I have now produced a statement of introduction, to send to writers who ask me to beta. It may not be enough, but at least I can feel that I have warned them that diplomacy is not my strongest virtue.