What about difference_update(), issubset(), issuperset(), __eq__()?

Raising TypeError looks reasonable to me. Operations with ordinal sets can raise TypeError for unhashable values.

>>> [] in set()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> set().remove([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> set().discard([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Unlike to set.__contains__ WeakSet.__contains__ returns False for unsupported types:

>>> [] in weakref.WeakSet()
False

Shouldn't set.__contains__ be changed to return False for unhashable values? Or may be make WeakSet.__contains__ raising TypeError for values that can't have weak references?