I've been reading a lot of Nietzsche and am curious about the first concept of "good". What was it like? Did it have the "noble" sense that the ancient Greeks had, such as might equals right, or was it more like the Christian "good" of love thy neighbor? Or something else entirely?
Zoroastrianism was not the first religion that put forward that concept. It just was the central concept: in the creation myth God and Devil made a wager. The fate of the world depends on if there will be more good or more evil thoughts, words and deeds (that triad is very important).
Of course there are a lot of parallels to Christian and Judaist and Grecian ethics, partly because they got influenced by one another, partly because Zoroastrianism was also the religion of an agrarian society.
But it will be very disappointing for someone who picked up the crass 19th century dichotomy between a docile but sociable Judaeochristianity and a proud and selfish Hellenism with it's many wily gods, as made by Frederick Nietzsche and by Matthew Arnold.
The parallels are: Thou shalt not steal or lie or kill. Thou shalt not covet other people's maidens. Honor thy father and thy mother. Thou shalt work in the sweat of thy face. There is one God and one Devil (and a plenty of angels and demons) and thou shalt have none before them. Respect thy elders.
Like Jesus Zarathustra was a reformer of a religion reminiscent of Vedic Hinduism or of the Kalasha faith. He emphasized it wasn't caste but your thoughts, words and deeds that determined your afterlife. A pious peasant goes to heaven. An evil prince goes to hell.
The original ideas are: Thou shalt not do ill to a dog, particularly if it is pregnant; thou shalt not litter; thou shalt not pour water into a fire; thou shalt not eat raw meat. Being around the sick or touching the corpses is also a sin (presumably caused by a plague). Fire is holy; thou shalt not burn corpses (as it is done in Hinduism).
As in the other Indo-Iranic religions menstruating women are evil.
Zarathustra explicitly mentions just war against heathens.
The nomad nations are an outright creation of the devil, along with vermin and with droughts.
Nietzsche wrote his Zarathustra because the European scholars were disappointed in all of the above after having heard Greek praise of Zarathustra as a great philosopher, astronomer and scholar. Thus the first translations of the actual Zoroastrian scriptures published by Abraham Hyacinthe Anquetil-Duperron were shouted down as a forgery or as a corrupt transmission. This discourse gave Nietzsche the opportunity to write of his own, more "profound", Zarathustra.
source: I.V.Rak, «čego ne govoril Zaratustra» («thus spake no Zarathustra»), Moscow: U-Factoriya, 1998