Many specs have a soft cap on one of their secondary stats: an amount they try to reach and not go over, because past that point, the value of the stat declines significantly.
About 50% of Frost's DPS stops benefitting from haste at 50% haste, when the global cooldown hits the minimum of one second: Frozen Orb, the selected bomb talent (the bomb's damage benefits from haste past 50%, its GCD can't go any lower), Ice Lance, and Brain Freeze Frostfire Bolts.
50% may sound like a lot, but with the raid buff and Frost Armor, the additional haste required drops to about 33.5%. A switch to Molten Armor is problematic due to the crit soft cap. A switch to Mage Armor is generally unwelcome because Mastery is a weak stat for Frost.
Even so, few Mages are likely to sit at 50% static haste or more. The problem arises when we mix in active haste buffs. Because Frost likes Haste so much, a Frost Mage will carry as much passive haste as possible, which means an active haste buff will put us way over the hit cap, sharply devaluing the buff. It's not a simple matter of the player carrying less haste: it's our most valuable stat, so we're usually better off stacking static haste and avoiding haste procs than we are reducing our static haste to accommodate the haste procs.
About 40-42% of Frost's DPS stops benefitting from Crit at 28% crit: Ice Lance and BF-FFB. Because these spells are cast only when they will benefit from Shatter, 28% crit rate gives them a 100% chance to crit against a boss. Against adds and spawns that can be frozen, the percentage of lost damage is higher, since Shatter affects more spells.
Frost does its best to get to 28% buffed passive crit, and thus receives sharply reduced benefit from dynamic crit buffs.
- Volatile Talisman of the Shado-Pan Assault: ~20.7% haste proc
- Sinister Primal Diamond: 30% haste proc
- Unerring Vision of Lei-Shen: 100% crit chance proc
- Tier 15 2-piece bonus: ~4.24% haste, crit and mastery proc
All of these items will be undesirable for Frost. Additionally:
- Breath of the Hydra: Locks us out of Frost Bomb, which is undesirable to some Frost Mages because Frost Bomb provides predictable BF procs.
This leaves only two trinkets as strong options for all Frost Mages:
- Cha-Ye's Essence of Brilliance (admittedly a very good option for Frost)
- Wushoolay's Final Choice
There is no alternative to the legendary meta gem; Frost Mages may be better off using the non-legendary mega gem instead. There is no way to avoid the two-piece set bonus (the four piece is strong).
This problem is only going to get worse in later tiers. Glyph of Icy Veins helps us avoid the soft cap with passive haste, but since we want to be as close to the soft cap as possible, it doesn't solve the problem of active haste being undesirable.
Frost needs a long-term solution to dealing with the soft caps. Until it has one, it will suffer from significant limitations in its selection of gear with dynamic buffs.
The question is: how can it be fixed? Convert haste in excess of 50% to something else? (Not crit, obviously!) Change the way Shatter works? Improve the value of Mastery so that Frost prefers it over the soft-capped stats? Post your ideas in the comments!