There are ways to balance the Warlock heropower though, e.g. by introducing new cards with card draw. If these are >= the heropower it would have a balancing effect on Warlock.
The actual nature of the problem is not that balancing Life Tap will be impossible, but that Life Tap is singlehandedly deciding how power gets distributed among the classes with every set. For every card that's designed with Life Tap
in mind, it gets that much harder if they ever want to touch Life Tap itself. The question comes down to: Do you think the game will be healthier with the bogeyman of Life Tap looming over the design of every set from here on in, or without it?
I would say without, in the same way I asserted that Flash would one day come to dominate the design of League of Legends champions (and it did). But that's not to say the game cannot survive, or flourish, just that things might be better if you nipped a potential problem in the bud then build your game around it to the point where it becomes the load bearing wall of the game itself.
In light of the Warlock discussion, I'd be interested to see what people's feelings are about which classes have been the strongest on average over time. For instance, Warrior is strong right now, but was relatively weak for some time when there was only one Warrior archetype, and that archetype could at times be weak to other meta decks.
This is from memory so it might be a bit shoddy. Also, I'm factoring both metagame shifts and the decks that existed at any given time. Thus, Miracle would dominate the Expert Set era even if it wasn't discovered until a year through, and Oil would dominate GvG in the same way. And this is only counting post-release and in no particular order.
Expert Winners: Rogue, Druid, Warlock, Warrior, Mage, Hunter (edit)
Expert Losers: Paladin, Shaman, Priest
Naxx Winners: Rogue, Druid, Hunter, Warlock, Warrior (edit)
Naxx Losers: Paladin, Shaman, Priest, Mage
GvG Winners: Rogue, Paladin, Mage, Hunter, Warlock
GvG Losers: Shaman, Priest, Druids, Warrior
BRM Winners: Rogue, Warrior, Warlock, Hunter, Mage
BRM Losers: Shaman, Priest, Paladin, Druid
Thus:
Warlock (4-0)
Rogue (4-0)
Hunter (4-0)
Warrior (3-1)
Mage (3-1)
Druid (2-2)
Paladin (1-3)
Shaman (0-4)
Priest (0-4)
There was only one point where Priest was ever one of the best and this was during the 8 mana Mind Control era back in Alpha/Closed Beta.