What were the key differences between the Roman Principate and the Roman Dominate?

I really like the description Peter Heather gives in his excellent work 'The Fall of the Roman Empire: A New History of Rome and the Barbarians'. I am not going to quote it verbatim, but in it, he describes the highly ceremonial nature of a recorded Senate meeting in the 4th or early 5th century. By this time, these senators were not engaged in debate, but chanting highly regimented blessings to the Emperor. Indeed, it was more akin to a religious ceremony than a meeting of governmental leaders. This is the end-point of Diocletians's change in how the Emperor was to be perceived.

Let's circle back here a second and explain these terms. 'Principate' refers to the government established by Augustus, whereby the position of Emperor was an unofficial de facto position rather than a de jure one. Emperors based their authority on their control over provinces that contained the majority (meaning all but one) legions in the empire. These legions owed their loyalty directly to the Emperor. At home in Rome, their authority to act as they desired was legalized by granting the Emperors permanant, at-large tribune powers. This gave them the ability to sit in and veto actions of the Senate, as well as sarosancity. This also included at-large imperium, which is the basically the ultimate power over life and death in the Empire, and more importantly, was required to lead troops. This included the city of Rome itself. A good summary of these powers is here: http://faculty.washington.edu/alain/CLAS.HSTAM330/AugPowers.html Now, the title Principate itself stems from the leader, the princeps senatus, literally the first man/senior man of the senate. The simplest modern way to think of it would be the President or Prime Minister. What all this really means is that the Principate can best be described in modern terms as a military dictatorship. The emperors, through their control over the military, took power, and retained power. However, legally speaking, there was no emperor, but merely a first citizen who happened to have the ability to execute anyone he wanted and controlled the entire military through personal loyalty. But on paper, still a Republic. While Augustus was keen to keep this appearance up, even living in a modest home, his successors slowly abandoned the pretenses of democracy; even by Tiberius's time (his successor) the Senate had become cowed and unable to operate independently for the most part.

Diocletian did away with all this. If we consider that ceremony Heather describes as the end-point of the change, the starting point was how Diocletian himself expected people to act around him. He sat not as a first citizen, but as a king. He expected people to prostrate themselves in front of him, to not speak directly to him, etc. He wore gold crowns and other lavish jewels to signify his position, and in general made no bones about what his position was.

It's important to note that the princepate and dominate were not hard shifts though. Many of the things that Diocletian adopted were simply logical conclusions to how the role of the Emperor had evolved. Imperial cults had been cultivated since the time of Augustus even, so the quasi-religious nature of the Emperor position, and the larger than life propaganda (Savior of the World is what Diocletian called himself) is not entirely different. But Augustus and most of his predecessors were still merely men. The thing is, men can be replaced. It makes sense that in the aftermath of the chaos of the 3rd century, where an Emperor lasting more than a few years was the exception, and men big and small rose to try to seize power on an almost annual basis, that a way to stabilize the system was sought. If you make the Emperor a larger than life figure, almost god-like, you might change that. Of course, that didn't happen, but to Diocletian's credit, revolts against Emperors by their generals did become much less. Some might link this to the idea of 'divine right' that medieval monarchs claimed. A cynic would say that this was simply to reduce the chance that a rival would seek your throne, particularly one without any familial claim to it.

Ultimately, the Dominate's powers were the same as the Principates in most ways. The main difference is the amount of power and prestige that he centralized directly in his own person (and then delegated to the Tetrarchs). In practice, nothing would have stopped many of his predecessors from doing the same thing.