I don't think Greyhawk is low magic, but I think it views magic differently as far as access and commercialization is concerned.
Eberron applied capitalism to magic. Low power magic is readily available for coin, and even the commoners have access to it. Magewrights, Dragonmarked Guilds, Elemental vehicles and such are all readily available to anyone with the gold.
Forgetten Realms has more centralized access, large metropolitan areas have access to magic that rivals Eberron, but your typical towns as roads are far less magical. Magic is the tool of the rich and powerful, from Lord's to secret societies to PC adventurers. The typical farmer, guard or merchant will only rarely see magic in his lifetime, if at all.
Greyhawk treats magic as a trade secret; it's for the elite few who learn it and those they wish to share it with, but it is extremely rare to see it given up for just mundane coin. Those who know magic know thier rivals well (and in the end, they are ALL your rival in some way) and use it to further thier own goals. Commoners know of magic, but only the most important ever see it and it is almost never just "for sale".
This scale would affect a lot of elements in the setting; spellcasting services, magic items for sale, frequency of magical traps and wards, etc. Eberron treats magic as a commodity, Greyhawk as special knowledge to further your agenda.