Rationale:

Several arguments were advanced for and against allowing set elements to be mutable as long as the ordering was not effected. The argument which swayed the LWG was one of safety; if elements were mutable, there would be no compile-time way to detect of a simple user oversight which caused ordering to be modified. There was a report that this had actually happened in practice, and had been painful to diagnose. If users need to modify elements, it is possible to use mutable members or const_cast.

Simply requiring that keys be immutable is not sufficient, because the comparison object may indirectly (via pointers) operate on values outside of the keys.

The types iterator and const_iterator are permitted to be different types to allow for potential future work in which some member functions might be overloaded between the two types. No such member functions exist now, and the LWG believes that user functionality will not be impaired by permitting the two types to be the same. A function that operates on both iterator types can be defined for const_iterator alone, and can rely on the automatic conversion from iterator to const_iterator.