Minecraft wiki entry for Development_versions, associated text: Items and entities will no longer keep unknown attributes.
Names of some attributes have been renamed to meet Namespaced ID requirements (i.e., lowercase separated by underscores instead of camel case).
Changed generic.maxHealth to generic.max_health.
Changed zombie.spawnReinforcements to zombie.spawn_reinforcements.
Changed horse.jumpStrength to horse.jump_strength.
Changed generic.followRange to generic.follow_range.
Changed generic.knockbackResistance to generic.knockback_resistance.
Changed generic.movementSpeed to generic.movement_speed.
Changed generic.flyingSpeed to generic.flying_speed.
Changed generic.attackDamage to generic.attack_damage
Changed generic.attackKnockback to generic.attack_knockback
Changed generic.attackSpeed to generic.attack_speed
Changed generic.armorToughness to generic.armor_toughness.