After listening to Mike Duncan's excellent "History of Rome" podcast, one of the repeated themes I noticed is that during periods when the Romans were more racist/classist/etc they had trouble. Their periods of greatest success were when they let people with the necessary skills and talent rise to the top regardless of their origin.<p>That said, they definitely had problems with failing to culturally assimilate immigrants during the late empire -- when "citizens" of the empire didn't think of themselves as Roman but instead primarily part of some other group.