To many McCullough is known for her Masters of Rome series. I particularly think that the first two books in the series, The First Man in Rome and Grass Crown were exceptional. The later novels cover the career of Julius Caesar and his heirs (both Augustus and Antony), which are rather well known to us. In contrast the life and times of Gaius Marius and Lucius Cornelius Sulla are not familiar to many modern people. In fact it is likely that their names would not ring a bell with the vast majority of people due to the decline of the classical education (which was the province of a narrow elite in any case when it was in vogue). But these were significant figures in their time whose influence echoes down the generations. For example, the Marian reforms depicted in The First Man in Rome arguably laid the foundations for the professional Roman legions which were to serve as the basis for the empire of the first few centuries A.D. Caesar nailed the republic’s coffin, but Marius built up much of its superstructure, making the armies loyal to their generals by opening up recruitment to those without other means of support. And Julius Caesar is a less surprising character if you are aware of the precedent of Sulla, whose vicious dictatorship he managed to evade.
Finally, of the peculiar things I recall about Colleen McCullough is that she wrote her massive novels in longhand (or at least her first few ones).