But what make Dark Sun so wrong?
Slavery is found in other D&D books.
Evil cities ruled over by evil rulers is found in other books.
Canabal Halflings? Well, plenty of others, like giants and goblins "eat people", not to mention dragons and such
A post apocalypse setting? Well this is a bit unique, but is it really "controversial?"
Dark Sun is unique among D&D settings in that the slavery is a cultural norm. It's not the result of a few evil people doing evil things; it's instead part of everyday society, and the only way to stop it would be to completely change the society from the ground up.
But on the other hand, Dark Sun is also unique in
many other ways. Elementals instead of gods and fiends, water and iron being scarce and worth more than gold, magic that destroys the environment, the prevalence of psionics, the completely different monster lists, the unusual new races and unusual takes on existing races. But for whatever reason, many people seem to think that it's the
slavery that's the most important aspect of the setting and that you can't have Dark Sun without it--despite it being probably the
least interesting part of that world.
Which is what
some of this controversy is about. For some reason, people really,
really want slavery in Dark Sun, even though it's not at all needed to make Dark Sun unique, it doesn't seem to
add anything to the setting besides, maybe, another level of in-game awfulness, and removing it wouldn't make Dark Sun less interesting.