I agree completely with your first paragraph. This is what I meant by the ease of ignoring an argument being a distorting effect of online or academic (i.e. paper publishing) conversation. In a real-time conversation or debate, this kind of plausible deniability doesn't exist as an option. I think my suggestion would help remove this plausible deniability and moves the online conversation form closer to real-time conversation.

We need a norm not for stating your last position, which is a burden on the person who has to reply, but for accepting irresponsible declarations

I think this is a good idea. Perhaps it can be implemented by allowing each commenter to set a status with the following options:

• I request a reply.
• No reply is necessary.

And having a norm that encourages selecting the second option when appropriate.

This is somewhat in conflict with mental hygiene: you shouldn't normally expose yourself to statements you can't sufficiently verify for yourself, but for this particular problem the balance seems to be in the other direction.

I can't make much sense out of this sentence. I expose myself to statements that I can't verify all the time, just by browsing the web for news and ideas. Do you want to try to restate this... (or not, it doesn't seem central to the issue at hand).

ETA2: Found a link for http://wiki.lesswrong.com/wiki/Epistemic_hygiene