I'm with DDT... If I wanted a set bonus and to see everyone running around in the same gear, I'd play WoW... However, I am not opposed to having slots in clothing/armor that we could craft additional pieces for to give our favored clothes/armor additional bonuses. This being a sandbox game, creating sets takes away a form of freedom and in turn reduces the sandbox feel. If a person was just looking for even more bonuses, I'd even go as far as to suggest worshiping an unnamed deity for a duration buff of either random or chosen from a list.
I do however understand that eventually spells or similar will be implemented, and to have someone in bone armor, for instance, hurling a fireball (example), might be a bit much... But if this were the case, slight penalties may be added to balance out the system, such as reduced spirit regen or damage output for heavier armor (think along the lines of the elder scrolls series). A good example for instance could be for each piece of grass armor hide skill increases by 0.10, though this bonus could be stacked - the wearer could potentially wear