I do wonder about slavery, too. It's generally the very brutish, abusive type (well, that's how it's been in history). But I also wonder about spinning it in an alternative method.
A culture where being a slave is revered. Or at least, being a good slave is something to strive for, at least among slaves. Not just a well-trained one, but one who's well behaved, who is "cultured" and socialized as a proper slave (the correct etiquette, the proper attitude and behavior). Slavery might also be an alternative to harsher circumstances: poverty, death, homelessness/worklessness, etc. So being a slave of a rich House is a better fate than farming dirt and dieing hungry. Slaves would be in a social class above the peasantry.
You also have the possibility of not slaves, but indentured servitude. Or slavery for a given number of years. Working for your freedom, etc.
Another route to take is slavery-ish type situations. To give an example, you have a Caste system. At the very lowest caste, you have the Untouchables, who are the whipping boy of society. You're born into this caste, you're destined to be a latrine mucker, a garbage collector, an undertaker, and other undesirable jobs. The highest respect you can get is still lower than a person in an entry-level position in the Caste(s) above you.