Re: 20% healing debuff on Permafrost and Improved Mind Blast:
Healing debuff effects (a la Mortal Strike), even if it's only 20% for casters doesn't address the issues of melee consistently trumping casters in the Arena or in BG's. Limiting healing effects on players strains healers and their mana pools. It doesn't make casters any more viable than they currently are. For anyone thinking that this will improve casters or move casters further into the limelight in the 'cleave' dominated Arena needs to think again. Warriors, Rogues and Hunters are all far better at applying the debuff, which, coincidentally is also far stronger.
Re: Caster viability
Making changes to allow for all casters to become more viable from an average player's perspective needs to center around instant AOE snares/slows and movement abilities, both on a reasonably short cooldown. The snare needs to be an AOE because multiple melee can truly lock down a caster now that nearly every melee has an interrupt, pushback and/or other forms of cc. In addition, movement abilities need to also be on a reasonably short cooldown to allow for escape. I'd argue that movement abilities should also almost guarantee a reasonable distance gained strictly because of the need for casters to actually cast.
Blizzard has already taken steps to move in this direction, but there are a few issues that need to be resolved, also coincidentally to classes that are very under represented in the Arena: Shadow Priests, Warlocks (Affliction/Demonology), Boomkins all need AOE snares. These classes have virtually no escape options if multiple melee get on them. Shadow Priests also do not have any short cooldown movement abilities that guarantee distance. Fade is not a solution.
While I realize that some classes are designed differently and function differently in combat, there needs to be a bare minimum with caster class parity or even homogeneity when it comes to dealing with melee. With classes scaling differently with gear/abilities as we move through the various expansions, it becomes more and more important that the bare minimums are met throughout all of the caster classes purely for survival, if nothing else.