In a word, everything.
As a consequence there are things that the Left will not tolerate.
The Left will not tolerate the independent exercise of force, especially when it is an act of self defense. The left seeks to disarm their subjects and will maliciously persecute those who dare raise a hand in their own defense.
Nor will the Left tolerate the existence and operation of the independent rational mind, for any factual refutation of any of the Left's beliefs will inevitably puncture their collective delusion of infallibility. Anyone who thinks for himself, let alone actually speaks up against the Left is publicly maligned and, where it is possible, physically assaulted.
In Rouge Kampuchea under Pol Pot the fear and hatred of independent thought was so strong that anyone who was suspected of being able to read was immediately taken out and shot. And as the Khmer Rouge ran up their bodycount leftist rags in America like the New York Times sat by and cheered them on.
Oh, and Thomas Sowell has his own ideas on the subject.