The Roman Empire begins where Alexander's conquests ended
The most obvious changes in the move from Hellenism is in the area of government. As a result of nearly constant civil wars between the city-states; the collection of small city-states, ruled by Archons, that had been Greece, were succeeded by an aristocratic republic. The Aristocrats also moved into the Senate. Roman conquests pumped more wealth into the Roman Aristocrats/Senators, solidifying their hold. In time, the simple Hesiod-ic Roman Farmer was replaced by slave labor in an attempt to further expand the wealth of the Aristocratic classes.
Augustus was among the most influential Roman rulers. He insisted that the provinces would not be ruled in a predatory manner, they were not to be plundered. Augustus ran the Empire by flattering the senators and allowed them to think they were making the decisions that ruled the empire. Augustus' successors weren't nearly as effective. In time, the military took over the empire and, as a result, the fiscal system collapsed.
The Greeks, meanwhile, were contemptuous of the wonders of Rome, but they were frightened of the Roman military. Fear lead to stasis; which, in turn, removed all incentives to achievement. As a result, Greek thought, art and culture declined.
The Romans demonstrated superior military might and superior social cohesion. The social cohesion was a direct result of the purity of Latin as a language. Everyone in the nation spoke the same language and the language served to identify citizens and exclude non-citizens and barbarians.
Art, literature, agriculture, commerce, and political science were still the domain of the Greeks. Rome was fascinated with these aspects of Greek culture. So, they co-opted these cultural products. Romans learned the Greek language. Roman poets copied the rhythms, styles, and metres of the Greek poets. Roman architects stole freely from Greek style. Roman Philosophers borrowed, amplified, elaborated, and subdivided Greek theories and called the product "Roman Philosophy." For everything other than good roads, laws, and armies, Rome looked to Greece as a model.
Certain social conditions in the Roman empire were not conducive to developing creative rhetorical philosophy. In the legal arena, the length of speeches, the number of lawyers, and the duration of trials were all reduced. Instead of defendants speaking for themselves, specialists spoke for them, so technical knowledge of the law superceded being a good speaker. What had been unconstrained political debate was severely constrained. Criticism of the ruling class could lead to execution as an enemy of the state. Epidietic rhetoric was also limited in that it could only involve praising the government or blaming convicted enemies of the state.
Thus, rhetoric became a matter of style over content. The public wanted to hear long, beautiful, eloquent language rather than substance. The Roman approach to rhetoric is borrowed entirely from the Greeks. It emphasizes the educational values of rhetoric. The Romans wanted to use rhetoric to educate the masses...thus the Roman theorists amplified the body of Greek rhetorical theory.
Differences between Greek and Roman Rhetoric an be summed up as follows:
For a complete discussion of the Roman Empire, head to the Roman Empire