Initially I agreed with this "Add a new comment for the same line is bad, because your comment will be added to the end of the comment list – so the reply is far from the comment it react too" but after check that when you double click on the commented line you see all the comments related with that line I don't think it is so bad to follow a conversation.
What I think still is needed to have a fluent conversation during the review process is the possibility of filtering comments to see quickly only new comments. I mean that you can choose from which date you want to see comments in the comments list. The dates to filter could be shown in a combo where you can select, no filter or one of the dates in which the revision changed its state or its assignee user. What it is important is that by default the list is filtered by last date when the revision changed its state or its assignee user.
In that way you can see very quickly what commented you other user on his last revision