Minecraft wiki entry for Development_versions, associated text: New armor attribute: generic.armorToughness.
Damage formula is now damage = damage * ( 1 - max( defensePoints / 5, defensePoints - damage / ( 2 + toughness / 4 ) ) / 25 ), i.e. increased toughness means less armor defense point reduction from strong attacks.
Diamond armor has toughness 2, all other armor has toughness 0. A full set of diamond armor, therefore gives a toughness attribute of 8.
New option enableWeakAttacks in options.txt.
Defaults to false. When false, the client will ignore attempts to attack when weapon cooldown is less than 75% recharged.
The effect was removed in the final release, however the option itself remains.