Hi Denny,
It's hard to give specific answers to your questions because a lot depends on context.
when is a personal attach or insult actually an attack or an insult
Partly depends on the language and tone of your post. For example if you were expressing disapproval because someone had rehomed their rabbits and was contemplating getting more then you could post:
a. I think your an evil cow with no concern for animals, I hope you drop dead.
b. Are you sure that's it's a good idea to get more animals so soon after you rehomed your previous ones? Perhaps you could consider fostering so that the rescue covers any expensive vet bills that arise. It's not really a good idea to go to pet shops because they don't health check.
a = attack, b = fine
Does this mean we have to slap them on the back and say well done
Nope, you've always got the option not to say anything at all. After all, if it's a done deed and they already know about rescues and the alternative options and went for a pet shop anyway, you yelling after the fact isn't going to make any difference.
Another option to consider is pm-ing people privately. If you've noticed a concern then it might be better to politely pm them and raise it rather than draw everyone's attention to it in public.
I think the problem with the 'Post, incite debate'
That's specifically about advertising. So no posting about your new rabbit product or posting a debate designed to promote it. e.g. My rabbits really chilly what can I do.... I know I'll use the new rabbit wooly jumper I invented.... you can buy them from me for 9.99
You're allowed to generally post and debate, that's sort of the point of the forum