The things we can't say, or can't say without a fight, are a pretty good guide to the things that have been removed from the domain of rational discourse. Other examples include abortion, feminism (though this was never in that domain), regulation, trade policy, taxes.
Commentator Joseph Goodfriend points out that "race" belongs in that list, though, like feminism, it is one of the subjects that was never part of rational discussion.
[2015.02.05] And economist Jared Bernstein points out that the distribution of income is another.
Any more to add to the list?