How was Roman art different from Greek art?Primary differences between the two styles/eras

"Roman Art" is a very broad term, and certainly the Romans
were heavily influenced by Greek art as well as Etruscan art (which was also influenced
by Greek art).  Perhaps the greatest difference between Roman and Greek
visual art is seen in portraiture.  Roman portraits of prominent public men
during the time of the Republic were quite realistic, emphasizing lines and wrinkles,
furrowed brows, and other signs of age and experience.  These portraits exemplified
visually what the Romans called gravitas, loosely translated as
"seriousness".  Signs of age showed experience, and the grave looks on the faces of the
men showed that they cared deeply about their public rersponsibilities.  Greek portraits
tended to idealize figures, emphasizing smmoth young faces and often god-like features. 
As the Romans came into increasing contact with the Greeks, many Roman artists copied
Grek styles but kept a certain amount of realism, with often jarring results: a portrait
bust of Pompey, for example, reveals an aged, realistically portrayed face with the
flowing hair of Alexander the Great!