The changes in ethics and morals through time and different cultures.
There are some talented actors among the cast, no doubt. There's also a plot, however tenuous the thread that holds it together may be. And the sets, costuming, and set decoration are aditionally visually quite satisfying. There's also violence, sex, buckets of gore, and a lot gratuitous nudity.
Taken with a bit of amused tolerance, and turning the brain off for a while, 'Spartacus: Gods of the Arena' makes for a very entertaining show for adults and mature older teens. However it is definitely not on for kids or tweens, and only for younger teens at a parent's discretion.
I'd suggest parents could use this show as a springboard for the discussion of ethics and subjective morality. Discuss about what is 'right' and what is 'wrong' according to different cultures, throughout different periods of history, and how every individual has their own personal moral code.
What other families should know
Too much violence
Too much sex
Too much swearing
Too much drinking/drugs/smoking