I enjoy the blog but don't read it regularly and have never (that I recall) commented on a topic. Prior to this thread, I would have assumed a forum thread on a blog topic would be okay but a forum thread on specific blog comments would not be okay. To me, it would be like starting a new post to discuss a comment you didn't like in another thread. I can't see that going over well with anyone.
That said, I can appreciate that when we're talking about the Admin's comment, which has been published as part of the blog, the water is murky. Is it considered part of the topic or part of the comments? I can see where those who enjoy discussing the blog would need that clarified.
As for the forum, obviously I do post here a lot. I would not want my comments shared as blog topics ("Forum Poster Jen recently said X with regard to topic Y, and her comment kicked of a lively debate because..." etc.)
As for forum topics, if the topic was made general enough that the original poster could not be identified, I wouldn't have an issue with it being turned into a blog post. I would hope the OP would be contacted first, either to ask permission or to give a heads-up. Again, I would be fine with this IF the post could be scrubbed well enough so as not to be identifiable by the OP's friends and family.
*Edited to clarify - I'm talking about general life/work/social topics that end up being great discussions, not very personal inter-relationship issues posters seek advice on, like with their DH's, in-laws, kids' teachers, etc. I don't think those can be generalized enough to be completely anonymous so I personally wouldn't want mine shared, just in case it get read by someone I know.