So on the one hand it's not a good idea to have threads in which people "end up talking to yourselves" - but in the case of anti-Obama threads people who disagree with what is said should refrain from posting in them, lest they "over run" them.

It seems to me there is an inconsistency there, somehow. Discussion should be about people who disagree exploring an issue from different points of view.
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I haven't seen any of the deleted threads, since evidently they were deleted before I got round to opening them, but I would hazard a guess that there would have been reasons for deleting them other than the fact that the person starting them was critical of Obama.

I have seen threads of various sort closed to further posts, generally for what has seemed good reason (not because of politics) but that's not the same as "deleted".