Like you, I have seen the variety of postings on various Book Discussions. I personally find the "notes" and "comparisons" of short segments of the book work best for me. If I have not read the book, these often will pique my interest in reading the book. When I read a "summary" of a book, it reminds me of a book review done in the newspaper and I often quit reading before the end of the post. In my humble opinion, to be defined as a discussion there should be postings relating to the book done throughout the reading. Which reminds me - I just finished chapter 5 of Once You're Lucky, Twice You're Good. I had an "aha" moment during the chapter and need to go put in my two cents on the book discussion.
Having attended many a face to face book discussion in my younger years, I remember the controversies being most interesting to me. I can see Glen's post about a summary of the book being more like a book review. I think if the first posting is a quick overview and the next postings trickle more into how people interpret the author or the work itself, that works best for me. I haven't taken a look at all the book discussions but a quick peek into the discussions should help Here I go...