Oh please. "Symmetrical" is completely dishonest. The creatures non-Priest classes tend to play tend to have even health, and Priests go out of their way to play high-health cards with a strong tendency for comparatively low attack.
There isn't anything stopping these classes from using these high HP or deathrattle minions along with neutral high HP minions:
Mage: Flamewaker, Mana Wyrm, Water Elemental, Antonidas, Spellslinger, Snowchugger
Paladin: Minibot, Keeper of Uldaman, Tirion, Eadric, Guardian of Kings, Quartermaster, Murloc Knight, Warhorse Trainer
Hunter: Web Spinner Highmane, Steemweedle Sniper, Gazrilla, Tundra Rhino, Desert Camel,
Shaman: Tunnel Trogg, Totem Golem, most of the totems themselves, Mana Tide totem, Flametongue Totem, Feral Spirits
Warlock: Void Caller, Imp Gang Boss, Doom Guard, Void Walker,
Druid: Keeper of the Grove, Darnassus, Druid of the Claw, Ancient of War, Cenarius, Druid of the Flame, Raptor
etc.
And then you add in: Zombie Chow, Scientist, Shredder, Creepers, Eggs. Argent Horserider, Acolyte of Pain, Unstable Ghoul, Sludge Belcher, Sky Golem, Cairne, Ysera, Toshley etc.
Lots of minions in the game straight up don't care about Lightbomb. It's a much weaker board clear than Twisting Nether and in most cases much weaker than Shadowflame as well.
Control Priest barely plays any minions at all which is why they can get away with using stuff like Excavated Evil, Auchenai Circle, Wild Pyromancer and Lightbomb. Hell the minions they do play if they die to board clears they don't really care either. Lightbomb is merely another tool in the Priest's arsenal to clear the board, alone it won't win the game but combined with the numerous other board controls Priest can play control style... and even then it's bottom of tier 2 and get clapped by numerous classes and archetypes.
: I actually think it would be cool if deck sizes and HP were increased in this game (maybe to 60 and 40, respectively), but everyone drew 2 cards per turn.
Aside from requiring a complete rebalance of the game, you also need to increase hand size too otherwise Mill strategies would dominate.