To defuse the "OMFG NOT WoW!" bits, to be fair WoW didn't do a gem/slot system first. They may have been one of the first MMOs to do it but hardly anything about the game was created specifically for WoW.
Also just to clarify it's been stated that you will be able to slot One piece of materia into each piece of equipment with 100% success. They've also been hinting at there being a way to slot more than one but it would have some kind of failure rate/negative effect.
Also Cairdeas, I think a better term would be passive materia effects as we already use the term Traits for things like Out of Sight, Prime Conditioning, etc... Just to avoid confusion mainly.