As far as I'm concerned the behavior in list B is correct and the behavior in list A is wrong. A is handy for being automatic, but it would be much nicer to have a B-only option, even if it means manually 'follow'ing each thread I post in.
As matters stand I cannot ever 'unfollow' a thread once I have posted in it. And when it devolves into a pissing match about who knows what fractional reserve banking is and what backing a currency means and who is ignorant and who is stupid, I desperately need to unfollow it.
EDIT:
Ah. There is a 'watchlist' link right below the one I've been using which has exactly the effect I was hoping for. I understand now.
I have been using it (rarely) for a different purpose, and remained unaware of what it did when there were new posts in "watched" topics.
Thank you for directing my attention to where I found it. I suggest changing the link text to "show new posts in your watchlist" to make it clearer what it does.
Yes watchlist is List B, I thought that's what you were referring to, asking for it to have the same functionality.