being a lock myself...
uh 1 thing..
free mount.. yeah it's 72-90g that's saved in your pocket for later that you don't need to use to get a mount
and for epic mount you need to spend smth like 300-700g which is about 150-200g less than the others(pala excluded)
and IMO the only thing unbalanced about the shard system is the damn thing that the shards don't stack!... it's pissy going in an instance when you need to carry alot of shards for teh just in case occasion and then needing to delete them when you get loot and in the end you have no shards, only loot and then you need to cast smth that requires a shard..
i mean the shard-spells like shadowburn may not be as good as mage's spells, but combined with Coil of death, Curse of Agony and Corruption the dam add's up quite well.. and because shadowburn is insta-dam it has to have some drawback like the shard req. since otherwise it'd be like the coil of death that has 3min cooldown.. that's just the way it is.. it's hard to balance classes since there's so many variables involved, especially with locks..
AFAIK the most powerful class is the shaman.. they are ridiculously strong.. i mean travel-form, hard hitter, aoe, totem's, healing, mail-wearing.. hard to beat that no?
EDIT
h yeah, and we get pets which gives us advantage on mages since they're alone, we get spells which hunters don't get and although i've only once defeated a hunter in pvp lock is one of the best pvp classes if played well.. only undead shadow priests kick everyone's ass..