And the rule is just that you can't direct your comment at the original poster, not that you can't talk about the subject being discussed.
I think for 99% of posters, this is a total non-issue. Just be a decent person and treat others with kindness and respect, and you won't have any recip ignores. There are very few people who get ignored, or especially recip ignored, without a reason. I have a couple ignores, and I bet it was because I used to be more confrontational in years past. I'm sure I ticked some people off. I'm sure I deserved those ignores. For the most part, it's up to you how many ignores you get, and how that affects your experience on the board.