After some thought I had come up with these points that define a good doctrinal discussion, including an online discussion, e.g. in comments on some post. I am not sure such a thing as this list is to be had online, but I have enjoyed many of these in person, particularly with family members.
It must be a discussion among people that:
(1) consider the scriptures to be the fountainhead,
(2) see the prophet and apostles past and present as providing a veritable river of truth and light to us, not as some group of guys with outdated opinions for us to batter with modern social trends,
(3) consider the doctrine worth discussing to be the core matters of the gospel, e.g. take McConkie's statement that the atonement is the least understood of all our doctrines as an example statement that our core doctrines are worthy of much more consideration, and
(4) recognize that understanding a doctrine usually means getting the big picture of it together, rather than debating some obscure corner to death.
And, last of all, the discussion itself must be edifying.
And no, one can't substitute just being polite to each other in place of any of these to have a good doctrinal discussion. Nor can one require this of only some of the speakers, and have a good gospel discussion. There is a reason there are so many times that Christ spoke of one matter or another to his followers in private away from the Pharisees and other critics, or on other occasions in a place that required some work to get to, such as up on a mount.