Imperial Rome and The Barbarians will feature at least one new unit type as well as several "specialty" units and leaders (Briton Chariots, Legionaries using the pilum, and Caesar, for example). Each of these operates within the confines of the original system rules, with only slight modifications to reflect their historical abilities and importance. So, rules-wise, if you've played Commands & Colors: Ancients, you'll have no trouble at all learning the few special rules for these new types of units.
The battle dice resolve combat quickly and the Command cards provide an element of luck that creates a fog of war and presents players with both challenges and opportunities for victory. Yet the tactics you need to execute conform remarkably well to the advantages and limitations inherent to the various Ancient units, their weapons, terrain and the history.