From an article on PCGamer.com:
In order to avoid this potential problem, Trion found a clever workaround: Rift’s Armor Sets are still defined by your class, but the extra bonuses given for collecting multiple the pieces of gear in that Set aren’t on the items themselves. Instead, those meta-bonuses will be applied through a new, compeltely separate item that will show up in the Character pane when 1.3 launches. They’re called a Synergy Crystals, and each one (there will be 32 different ones at patch launch) gives bonuses to different stats–but only once you’ve collected enough pieces of your Armor Set. Alone, the Crystal doesn’t do anything, but with each Set Item that is equipped, it provides more and more stat bonuses.
For example, a Warrior Riftblade will want to get a Crystal that gives bonuses to Crit, Hit, and Damage, while a Warrior Warlord will be searching for one that enhances his Health and Armor. They’ll both be rolling on the same Plate armor pieces to collect the same Armor Set when they drop from the new Hammerknell content, but the two players will be using them to activate and empower their own, distinct Synergy Crystals–getting completely different bonuses out of the Armor Set. There’s a Snyergy Crystal for each Soul in the game, so there should be at least one good option for every single combination of Souls.
It seems there will be another factor in creating and building your character. It is yet to be seen if these armor sets will strictly be from Hammerknell or will be inserted into current raid or dungeon sets.