One of the new 'features' of WotLK is that as of beta build 8788, almost all spells that can be downranked will now cost a percentage of base mana. Blizzard has already indicated that they will be looking further into new balancing issues that will arise from this, but I'm not entirely sure this change will be beneficial to anyone with a mana pool. Of course, we're still in beta, and things can change and the interaction between new gear, new talents and changed stats such as spirit have yet to truly be seen.
My biggest issue with percentage based spell cost (PBSC) is that as casters, we are inherently limited in effectiveness the longer the fight, be it PvE or PvP, goes on. Rogues and Warriors have essentially limitless DPS and as such, are incredibly effective at all points in an encounter. An oom caster, while at range, becomes little more than an additional mana sink for their healers, as they are unable to contribute.
Let's describe a scenario: A mage/healer team encounters a melee/healer team. The mage goes oom, trying to pressure the opposing healer into spending alot of mana and uses poly (rank1) to conserve the teammate healer's mana. This strategy can work even when the mage is nearly oom and waiting for a mana gem as poly (rank1) and frost nova (rank 1) cost little to no mana. Let's change poly to a PBSC and now both poly and frost nova are prohibitively expensive. CC's become harder to land and due to the mage being oom, the mage's healer must spend more mana to keep the mage alive while the opposing healer can drink in relative safety due to the lack of pressure. If melee characters had spells that had a reduced rage or energy cost for spells such as hamstring (rank 1) and had a limited amount of rage or energy, you can bet they would never max rank either.
My second biggest issue is that it completely hoses utility spells that were used in PvP only as rank 1 due to mana conservation issues. For a druid, spells like moonfire (rank 1) to kill totems, faerie fire (rank 1) to keep rogue's from vanishing are integral parts of managing opponents. For a mage, frostbolt (rank 1) and cone of cold (rank 1) are amazing control tools used to keep people slowed for minimal cost. Other classes also use lower rank spells to conserve mana while providing utility other than just damage/healing. Just a few off the top of my head:
Stacking debuffs with shadow word: pain, faerie fire, winter's chill
Stacking trash buffs with mark of the wild and thorns to protect innervate from dispels
Spamming healing wave for ancestral fortitude
Spamming arcane explosion, blizzard, hellfire, rain of fire, consecrate, magma totem to expose stealthers
Save mana on expensive totems (windfury totem) while providing essentially the same effect
The list goes on and on. You can see the skill level difference when you see top players use various rank 1 spells as compared to users who max rank spells. Knowing when to max rank and when to downrank is a skill that should be learned and only adds to the complexity of high end play. In addition, this means that offensive and defensive dispels are now worth their weight in gold as any buff/debuff will now cost its maximum to reapply.
My third problem is a bit more esoteric in that PBSC encourages bad play. Let's take a PUG 5 man for example. Tank gets hit hard before cc lands and the healer pops a max rank PBSC heal and suddenly gets aggro. Was this preventable? Sure, the tank could have gotten aggro better, or the cc could have been executed earlier, or the healer could have downranked a heal, but when you are essentially penalized for downranking, to the layman healer, why would they use anything less than max rank? Joe Average doesnt know about downranking and cannot see anything beyond, "ooh big numbers!" Currently, you can make a case of, "downrank because it wont generate aggro and you wont spend mana." but it will be much harder to explain "downrank because your aggro will be less, but you'll waste mana while doing it".
Lastly, I'm wary that Blizzard will forget to consider something that could be highly impacting with this change and we'll all be hosed until a hotfix can come through.
Of course, we're still in beta, and things can change and the interaction between new gear, new talents and changed stats such as spirit have yet to truly be seen.