While the OP's description of rolling 18 dice with Hand+Battle Fury has since been nerfed, the concent of a "kill threshold" remains one of the most important concepts in the game.
When my Mage is running around the board with 18 damage out of 32 health, that health number (14 hp remaining) helps to tell me how easily I can be killed.
- If I have no Armor, no Aegis, and no facedown enchants, you can throw Hurl Boulder x 2 with a ~56% chance of killing me in a single round.
- If I have Armor +4, Aegis 1, and a facedown Healing Charm, you cannot kill me in one round with Boulders alone.
- If you have three Butchers and a Sniper all in range to hit me, and you have the mana for two Boulders, I am still within the "kill threshold" despite Armor and Aegis.
The existence of a "kill threshold" really limits my tactical options. If you are also within "kill threshold", the advantage goes to whoever has initiative or is able to kill in the least # of actions. However, if only one player is within "kill threshold", he's playing at an incredible disadvantage. All of his moves
must be defensive in nature, or else he will probably lose the game
this round.
Because of this, getting your opponent within the "kill threshold" while being far away from it yourself, is the next best thing to killing him outright. This is a crucially important concept for the early game. If both players have 32+ health, it is impossible to do that much damage in one round. However, every point of damage you do, every Rot and Burn and Ghoul Rot, and every creature within attack range puts that Mage closer to the "kill threshold".
Ultimately, the first 31 damage don't really matter - a Mage with 1 health can still hit you just as hard as a Mage with 32 health. However, the first 20-ish damage force the Mage to change his gameplay (from not being afraid of death, to being very afraid of death) and this gets you in position to finish him off.
Kill threshold also matters for individual creatures, especially the expensive ones. If you are rolling 5+4+3 dice against Brogan (4 AC, 11 HP) it is very unlikely to kill him in a single round of attacks (~6.3 damage per round), but very likely to kill him in two rounds of attacks. This means that an opponent who wants to keep Brogan alive will need to heal him every round - for either 5 or 9 mana depending on how well you rolled last round. That's a huge mana investment to offset free melee attacks.
On the other hand, a Steelclaw Grizzly (3 AC, 15 HP) is likely to take 3 rounds to kill, which gives your opponent time to take other actions instead of constantly healing his big creature.