When argument theads get too deep you can't indent them anymore so it becomes difficult to tell which responses go to which arguments if more than one person responds.  When the indentation gets too deep, just start the next response back over on the left, but display some kind of indicator like this .  For example, currently deep threads look like this

Bla bla bla bla bla blabity bla

        Bla bla bla bla bla blabity bla

                Bla bla bla bla bla blabity bla

                        Bla bla bla bla bla blabity bla

                        Bla bla bla bla bla blabity bla

                        Bla bla bla bla bla blabity bla

I propose you make them look like this...

Bla bla bla bla bla blabity bla

        Bla bla bla bla bla blabity bla

                Bla bla bla bla bla blabity bla

  Bla bla bla bla bla blabity bla

            Bla bla bla bla bla blabity bla

                 Bla bla bla bla bla blabity bla