Just think about it like this for a minute. You are a priest, paladin, mage, or a non-enhance shaman. Once you are at zero mana with no more mana restoring CDs, what can you do assuming the other team doesn't let you drink? The answer is, basically, nothing. Which is a lot like being dead really. So this means every one of those classes really has two health bars. One is their hit points and the other is their mana. Reduce either to zero and they are out of the fight. But look at the differences:
- Hit point pools are larger than mana pools yet mana drains deal comparable "damage" (less, but still comparable).
- Hit points can be healed, mana cannot (beyond a few CD limited and counterable abilities).
- You must spend mana to do anything useful, so even when you aren't being focused, one of your health pools is always being depleted.
- There is no damage reduction or absorption abilities that stop mana drains: PW:S, Shadow Ward, PS, all do nothing.
The bottom line is mana drains as a mechanic are far too strong. They shouldn't be removed because they are a very valid counter-tactic. But right now they are an overpowering strategy that is effective so quickly, there just isn't a reason to try to burn down a non-warlock caster when nearly every ability on every healing class protecting your target is designed to work against damage and not mana drains. Use mana drains and their team might as well not even have healers.
But I think a very reasonable fix is already available and just waiting to be implemented. Resilience could easily be made to affect mana drains of all flavors. As a bonus, using resilience as a counter will have zero effect on PVE encounters requiring anti-mana tactics. Now exactly how much effect resilience should have is a good question, but I believe the full 2% per 39.4 rating is the correct amount. Even when nuking mana only 75% as fast, mana drains will still be dangerous. But that extra time will hopefully keep them from become the sole strategy behind every top-level arena team.
Ultimately, mana drains are crowd controls of the permanent variety and without any diminishing returns. Much like Wizards of the Coast did with Magic: The Gathering when it came to Land Destruction, mana drains need to move away from a core strategy and become more of a spot application tactic. Land Destruction wasn’t fun to play against in MtG and mana drains aren’t fun to play against in WoW.