Minecraft wiki entry for Development_versions, associated text: Started to move over various things to a new attribute system:
Item attributes using NBT tags:
Attributes can be additive, subtractive and multiplicative using a set value.
Some items have default modifiers in place of previously hard-coded attributes.
Examples of possible changes players could make: A bow that slows the holder, a helm that buffs health, etc.
Entity attributes:
These properties can now be changed as easily as any other NBT attribute.
Entity speed, base health, base attack.
Other properties like weapon damage/bow draw speed/sword swing speed are yet to follow.