Scott, I think that if the thread is reopened it needs to because YOU are comfortable with reopening it.
At the end of the day, I care more about your comfort and contributions as a moderator than whether any single post should be moderated (open or closed).
If you do choose to reopen it, I would suggest that you consider adding a statement that the thread has sparked a good discussion amongst the moderators and boards and helped us to better understand when and when not to moderate.
One thing to consider in the future, if a thread "may" go bad, is rather than closing it for the moderator to make a statement on the thread letting everybody know to please keep their comments within bounds, as the thread has a potential to go off the reservation.