The saddest thing about this set is when you compare it to the mage set:

Plagueheart: 227 Stamina 155 Intellect
+292 Damage 4% Hit 6% Crit
Frostfire: 166 Stamina 187 Intellect 51 Spirit
+329 Damage 6% hit 5% crit

Then compare the set bonuses:
Plagueheart:
2 - Reduces cooldown and delay on your Curse of Doom by 10 seconds. (semi useless in PvE, totally useless in PvP)
4 - Your Shadow Bolts now have a chance to gain Vampirism again targets afflicted by Corruption, healing you for 270 to 330. (because we're going to be using SB and Corruption a lot in a Shadow Resistant instance)
6 - Your spell critical hits generate 50% less threat. (which is actually less than the 8/8 Nemesis bonus if you even have as much as a 20% crit rate)
8 - Reduces health cost of your Life Tap by 12%. (which becomes our only means of mana regeneration... and this is less than the 8/8 Felheart bonus)

Frostfire:
Set Bonuses
2 - Reduces cooldown on your Evocation by 1 minute. (which increases their mana regeneration by 10%)
4- Gives your Mage Armor a chance when struck by a harmful spell to increase resistance again that school of magic by 35 for 30 sec. (pretty awesome passive for both PvE and PvP)
6 - Your damage spells have a chance to cause your target to take up to 200 increased damage from subsequent spells. (Holy Wrath of Cenarius Batman! This is easly another 6-10 dps)
8 - Your damage spells have a chance to displace you, causing the next spell cast to generate no threat. (as anyone who has used Fetish of the Sandreaver can tell you, this owns 50% less threat from crits).

Added to the new mage patch details, it means that mages will be able to cast more, longer, and with significantly less threat and more survivability than warlocks. Good thing we have 600 more health so that it takes mobs maybe 1 more hit to kill us...