Starting again to read the Barsoom series and after spending the last week pouring over Carcosa, I'm beginning to realize how much I love the swords and planet genera. Maybe swords and planet isn't quite the right word, but I've been discovering two specific setting aspects I really enjoy:
Ancient Technology - I love the idea that what we would normally was futuristic technology is actually ancient, so ancient that it is no longer complete understood, or even misunderstood.
Not so Nice Magic - Magic in D&D is just a little too clean, a little too nice, and a little too common for my tastes. One of the reasons I enjoy Carcosa so much is because there are non-rules reasons for characters not to be using magic all the time. They don't need to be limited in the number of spells they can cast per day because 1) rituals are so difficult to use (need to have all the right ingredients, at the right time, in the right place) and 2) it's just plain evil. If you are willing to use rituals in Carcosa, it says a lot about your character (whether you sacrifice children or study the ancient ways of the extinct Snake-Men and have decided to use banishing rituals exclusively).