My current theory is we won't be equipping individual pieces of armor, rather entire sets. It would solve the old hodgepodge ugly armor sets that result when you're wearing red gloves, a purple hat, green pants, a blue body and and brown shows. This is an MMO, not a disco strobe light.
I dunno about this - unless the entire set has a bonus that's good for all abilities you're currently using, it doesn't make sense NOT to mix and match. I mean, let's say you're a healing Conjurer. Why wear an entire set of matching armour that has one really good bonus for your healing, but the rest is more dps based? It's severely limiting to force entire gear sets on people. Unless they're going to totally simplify the gear system. But having 10+ gear slots (head, hands, feet, waist, back, chest, rings, neck, weapon, ammo etc.) seems to be fairly standard across the MMO board.
Don't forget, crafters are going to be able to dye gear - hopefully ANY gear, not just gear they make. So the mismatched gear problems will hopefully be a thing of the past. I mean, if they provide us with a decent amount of inventory space, it won't be such a big deal to have an "in the field" useful but ugly set of gear and an "around town to look cool" set. ;)