How are “communist” dictatorships different from other forms of authoritarianism?
Communist authoritarian governments are only different from other sorts of authoritarian governments in the ways in which they try to control the economy. To understand this, let us look at Russia during and after communism.
In both cases, Russia's government has been authoritarian to some degree. It is not particularly safe to criticize the government too much today (though it is much safer than it once was). But the real difference comes in the economy. In the Soviet Union, the government completely ran the economy. Companies, such as they were, were run as bureaus of the government. Today, companies are run more or less as companies in the West are run. The government has close ties to them because the people who own the companies are so rich. But the government does not run the companies. This is the major difference between communist and other authoritarian governments.