Horde: Blood, sweat and maybe sawdust
Alliance: Soap, probably a mix of lavender, violets and patchouli(though only if there are nelfs or draenei near)
CC/CE: Forest
AD/AC: More soap, mixed with horses
KotEB: Blood, Iron and corpses
Kirin Tor/Violet Eye: Even more soap, this time mixed with incense
Wyrmrest Accord: Burning stuff? Soil? I guess it depends on the type of dragon