That's interesting because the Genealogy of Morality was his greatest contribution as far as I can tell.

Is there something in particular that feels most egregious about master/slave morality or do you have some reading attempting to reject the ideas?

You will see very quickly whether or not the person understood Nietzsche to have been writing almost entirely prescriptively or descriptively. The former almost always holds a caricature of Nietzsche's thought as the edgy truth.

GoM is an interesting work on his ideas about the historical evolution of social norms and the psychological motivations behind them in many cases; in either it or TGS he says that a really comprehensive work on the evolution of norms throughout human history and different societies in different conditions would be invaluable, and that his work is only sort of a first step on an enormous undertaking. Unfortunately, we never much took it up. Maybe that's inevitable, since Nietzsche in a rare normative moment says the exception should not become the rule; we don't want a lot of people to not take their norms so seriously they reflexively reject serious consideration of their contingent character. If they do, you don't have much of a society.

It's certainly very hard for people to seriously think about these problems. Plenty of very smart people take thinkers like Rawls seriously, where Nietzsche's work clearly shows it to be without any serious foundation. You could of course still make a claim for moral realism, but it requires the acceptance of ideas the same people are usually allergic to.