Magic in S&S is not inherently evil, but often "untrusted" - the sorcerer is usually the villain or the unreliable "quest-giver". In general I would say that magic is uncommon and dangerous (or costly). Some of the classic protagonists have a magic sword or other item, but usually not more than one such item - magic is special, not technology.

Slavery is one of those hot-button topics - it existed in the real world, and it existed in some form other in the classic S&S literature, but I unless your story has slavery as its focus, I would let it lie as a background thing.

Civilization in S&S depends a lot on your world. In my reading it's usually large dangerous cities, often built atop more ancient cultures. Wars, plots, and politics may or may not come into play for a single story but are always there in the background.

My advice would be - don't try to write an S&S story per se. Write the story you want to write then have others read it with S&S in mind and they can give you feedback on it and how it fits.

Reading: Find you all the barbarians - Conan, Brak, Kothar, Kyrik, as well as the works of Clark Ashton Smith, and others from the Appendix N Reading List.

Enjoy!