You would really lose the 'discussion' aspect of things by turning it into a 'say once' system.
People in complex situations almost never behave exactly as you'd expect them to. An artificial limit to discussion might have terrible unintended consequences. What about the problem of someone asking a question in their comment.
"Hey, wait a second, what did you mean by X?" [I'm sorry you're prevented from replying in this thread]
I know the time delay thing was a good start. I just think it needs to be extended. It seems that threads lose value as they get deeper.
Oh, and I just thought of #8: It encourages you to get back to work since you won't be replying. That's something everyone needs to consider.
2. Trolls do not care who replies. This can avoid the argument between the OP and the troll, but does not prevent the latter to post the snarky comment in the first place. Some people just want to express their frustration attacking someone, they do not care about replies. Moreover, the lack of chance for the OP to reply could even incentivize the bad behaviour, giving the flamer a sort of "revenge" weapon. People often find gratification in silencing others.
3. Some discussions give value to the community because they happen in a short lapse of time. If someone needs 2 hours to reply, then there is a chance fewer people will see it.
4. As I said, trolls just want replies, from anyone. Flaming the full community could even be more satisfactory to him.
5. Unfortunately often there is no way of doing this because some users do not have their address in their profile.
Also, if you can edit your comments, you're opening the door for twisted threads where replies are in one block per person with "@person timestamp," although that's easily fixed by only allowing editing for a short while.
Limit the number of times one person can post in the same thread to once (or maybe twice), either from the top node or a sub-node. Example:
PersonA says, "blah blah blah..."
PersonB says, "You're an idiot because blah blah blah..."
PersonA is locked out from replying to PersonB.
This will do 7 things:
1. It will eliminate argument strings between 2 people.
2. It will remove the incentive to flame because your flame can't turn into an argument.
3. It will encourage people to think through what they have to say, anticipate counter-arguments, and address them in their original comment (or with an update in the 2 hour edit window).
4. It will foster a greater sense of community by encouraging people to stick up for each other. (PersonA made a good comment, PersonB trolled, PersonA can't reply, so I will.) PersonB will get the message much better when confronted by the community instead of PersonA. (Of course, if no one sticks up for PersonA, then maybe they really were wrong.)
5. It will encourage 2 people to take their legitimate discussion (of little interest to others) off-line.
6. It will reduce the length and indentation of threads making them easier to read.
7. It will encourage everyone to slow down and think a little. Those who don't will stick out (not in a good way). Trolls will get more satisfaction somewhere else.
(This is already working by accident. I have gotten so busy that I only visit hn in short bursts. Often, when someone challenges me, I don't reply because I have left and returned much later. But others have replied for me. The resulting thread is invariably better than if I had replied myself.)