Oh, and by the way, from what I understand it looks like each class has a armor set that is open. By "open" I mean it can be used with any skill set as long as it is in conjunction with the 1 or 2 abilities that the armor set buffs. For the barbarian, the Immortal King only buffs 2 or 3 abilities, and so thusforth, can be used with 2 or 3 other abilities, making it an "open" set/build. Might of the Earth seems very closed, but from what I hear, it is one of the best armor sets because of all the damage it puts out.
Here's an illustration:
Armor sets and the abilities buffed by each:
Immortal King: Wrath of the Beserker; Call of the Ancients
Raekors Charge: Furious Charge; Battle Rage; Earthquake; Sprint; Ancient Spear; Whirlwind; Seismic Slam; Rend; Hammer Of The Ancients
Might Of the Earth: Leap; Ground Stomp; Earthquake; Avalanche; Ancient Spear; Seismic Slam
You should check out my Barbarian build. I have just realized how armor on this game works best, and I am enjoying a VERY strong barb in comparison to what I have been before I understood how the armor sets work.