The following specs can de-magic/poly immune:
- Healers
- Druids
- Shadowpriests
The following specs can de-curse (Hex):
- Shamans
- Mages
- Paladins
The following specs can de-magic/fear break:
- All healers
- All Undead
- Warriors
- Shamans
- Priests
- Death Knights
- Monks
- Paladins
Also, regarding Hex breaking on damage, it breaks on twice the damage that the easier-breaking fears break on. Outside of MLS and Shatterplay, not many serious comps combine two of Mage/Druid/Paladin - which means so long as you are willing to always throw your Hex on the de-curser, it should always go full duration. Similarly, so long as you throw your Poly's on the enemy healer or Spriest (if both are present) it should always go full duration. How many Fears do you think go full duration in MoP? Maybe as much as half? Maybe more like a third?
The healing factor on Polymorph isn't a concern to frost mages, they aren't a pressure class - when they set up their CC, given the comps they play - they can burst a full-health opponent down. By contrast, Affliction and Shadow are reliant on getting a target low before they go for a kill - if fears healed people then pretty much every pressure comp out there wouldn't work (with the exception of Spicy Chicken, which is the only pressure comp without a fear class).
Frost Mages don't have DoTs - so the risk of Polymorph breaking on damage is "don't poly then ice lance your own sheep" - where as DoT classes (warriors included, via rend/deep wounds) would automatically break their own CC if Fears didn't absorb some damage: the balance is in finding a value where you can still Psy Scream / Intimidating Shout / Howl of Terror / Fear (as a peel) without it instantly breaking, and without it becoming a semi-stun effect. Classes which are totally reliant on DoTs need CC's that don't break instantaneously on damage - for the same reason, Balance and Ferals get Roots and Cyclones - neither of which break instantly on damage either. If a mage breaks his own poly, it's his mis-play in attacking something he just CC'd - if he's playing with a class that has DoTs, he has a glyph to wipe DoTs on application.
The damage that fear absorbs is random, it should be a fixed value (ie. 10% of max health) - and should be equal for all fear effects (it's not). The real thing that people hate about fear though isn't that it absorbs DoT ticks, it's that it fears you into dumb places - all fears should cause targets to tremble, rather than running around blindly. Lastly, there are probably too many ways to break fear at this point, but that should probably only be looked at after the first two things are adjusted.