Magic in the primitive, Dawn of Civ campaign is going to be quite different than what we are used to the standard late-medieval campaign milieu. Even in our standard late-medieval milieu, we often conceive of magic as a science with ancient roots and substantial development. Stripping away this implicit-ancientness of the magical arts, and returning magic to its primitive foundations, results in a unique "new look and fee" for the magic using class.
To begin, we must delineate the exact domain of magic: the harnessing of the natural magical energy of the material world. The prime characteristic of magic users is INT, because magic is a science, like chemistry and physics. Rather, magic is an applied science, like engineering, built upon the laws of chemistry and physics. In the primitive setting, there are no "schools" of magic, although individual magic users might specialize in particular areas of magical research.
The Creativity of the Magic Process
The magic of the primitive world has not been encompassed by books containing standard spell lists. Magic use is open and creative. Spell creation is not limited by level. In fact, the creation of spells and magical formulae is what defines a powerful magic user. An intelligent magic user is open to harnessing new magical powers at all times, in an on-going investigation of the magical world itself. He can benefit from the spell books of other magic users, but he is not limited by them.
For example, the magical effect of Charm Person could be created from the voice box of the Siren, and contained in various forms, such as a powder, a wafer, or a potion, which when consumed, would allow the magic user to charm via his song.
A Charm Person spell created from the body of a Will O Wisp, however, would have a slightly different power and method of application, perhaps better suited for a wand which would release magical lights, charming its viewers.
Compared to Clerics and Psionics
Magic does not deal directly with the spirit realm, that is the domain of the cleric. Nor does it rely on "mentalist powers", that is the domain of the Psionic. We can expect that there would be very little crossover in the powers of the cleric and the magic user, because their power is based on different realms: the spiritual for the cleric, the material for the magic user.
Nor is magic simply a synonym for "fantasy wish fulfillment", meaning, a way to satisfy any imaginary possibility. If a magic user is to make some magical effect, it must be based on the sound principles of magical science. A magic user is not a god, who can make anything happen, a la the "wish spell". Gygax seems to regret inventing such an illogical spell, judging by the ways he is constantly limiting its power and scope.
The Efficacy of Power Words and Symbols
Power words and symbols were learned from the magical creatures of the world, literally imitating their sounds and languages. However, for humans, the sounds by themselves are ineffectual. The sounds must be accompanied by the magical substances. For example, a magical powder or elixir, inhaled or drunk, THEN the power words spoken, with the magical substance covering the throat, releases a powerful burst of magic.
Humans themselves are mundane, non-magical creatures. Only when their actions and words are accompanied by magical substances can they create magical spells. As a corollary, we can see that magical spells are not "memorized" for efficacy. Spell use on a daily basis is limited to the magic users CON score, because channeling magical power through his body is stressful and draining. Magic stored and released through magic items, such as wands or scrolls, do not drain his bodily power, and are not limited by his CON score.
The Hatred of Magic Users
Magic Users, and humans in general by association, are hated by magical creatures, because of the exploitative method of magical ingredient harvesting. Boots of Elvenkind, for example, are not boots created BY elvenkind, but are boots OF elvenkind. As you can imagine, elvenkind does not appreciate having their foot skins harvested for the making of human boots... Nevertheless, humans value such boots greatly, as they allow their wearers to move silently through the forests.
The only humans free from this stigma attached to magic users are Barbarians, who steadfastly refuse to engage in magic or use magical items.