Warlords of Draenor is making alot of changes to player and item stats. Some stats are being removed, others are being altered and there's even a whole bunch of NEW secondary stats being introduced as well as an entirely new type of stat. I thought it would be a good idea to lay out all the changes in a way that makes sense, because it's actually a bit confusing.
Reforging is being removed from the game, and less items will have gem slots or be able to be enchanted but gems and enchantments themselves will be more powerful and have more options.
Primary stats on 'main' armor pieces (Helm, Shoulders, Chest, Gloves, Legs etc) will transform to match your current spec. For example a Mail helm drops from a raid boss: to a Hunter it has Agility but to a Resto Shaman it has Intellect. Switching specs will also cause that helm to change to suit whatever spec you are currently playing.
It was mentioned that Primary stats will not appear on certain items such as Necks and other 'non main' slots.
Tertiary stats are extra bonus stats that can appear on conventional items in addition to Primary and Secondary stats and will be somewhat rare. It is unlikely you will be able to stack Tertiary stats due to their rarity. No word yet on if it will be possible to gem or enchant them, but this is also unlikely.
It is possible that certain Tertiary stats will only appear on specific item slots; eg. Speed will only be on Boots, Lifesteal only on Gloves etc but Blizzard are still undecided about this.
Hit & Expertise are being removed completely.
Parry & Dodge are being removed, but players will still be able to parry and dodge attacks via their abilities.
Stamina, Agility, Intellect, Strength
Critical Strike, Haste, Mastery, Armor, Spirit, Amplify, Multistrike, Readiness, Attack Power, Spell Power.
Avoidance, Cleave, Lifesteal, Speed, Sturdiness.
What the new Secondary stats do:
Most gear will still have a standard amount of Armor as it always has, but bonus Armor can also appear on certain Weapons, Rings, Necklaces, Trinkets and Cloaks. Armor is the Tanking stat.
Spirit can only appear on certain Weapons, Rings, Necklaces, Trinkets and Cloaks. Spirit is the Healing stat, and will continue to work much like it does today, although apparently you won't need as much.
Amplifies all of your other secondary stats by X%.
Grants your attacks an X% chance to deal additional damage.
Reduces the cooldowns of some of your main abilities by X%.
Attack Power & Spell Power:
Pretty self explanatory. Can appear on any gear just like it used to in the Burning Crusade and before.
What Tertiary stats do:
Reduces damage taken from AOE abilities by X%.
Grants your attacks an X% chance to hit additional nearby enemies.
Causes every hit to leech X% health from enemy targets.
Increases your movement speed by X%. Stacks with other speed increases, but will have a (generous) cap.
Reduces durability loss when you die by X%.
So, we are going from just a handful of secondary stats to 10, plus 5 possible tertiary stats. This is going to make gear in Wow ALOT more interesting. It may also cause some theorycrafters' and simcrafters' brains to implode... or at least get very annoyed.
I've gathered this information from Blizzcon and Twitter and am relatively sure it's all correct. But feel free to tell me if I've got something wrong or left something out.