As many of you know from panels at last year’s BlizzCon and posts here  on the forums since then, Cataclysm will bring about major changes to  familiar character stats such as Intellect, Armor Penetration, Defense,  and others, ultimately designed to make the effects of stats more easily  understandable and make gear choices more interesting. As these changes  will have a significant impact on how stats work and relate to one  another, today we wanted to offer you a closer look at exactly what’s in  store and explain some of the rationale before Cataclysm arrives.
The  most obvious question these changes raise is "Why are stats being  changed, and why now?" As the game has matured, we've run into  increasingly complex issues with the current stat system. Many stats are  inherently confusing, and the way they interrelate can feel convoluted.  Attack Power, for example, currently translates to damage, but so does  Armor Penetration. Defense provides five different statistical benefits  of varying utility. Mana regeneration involves understanding multiple  stats and rules and often ends up being irrelevant anyway. In addition,  the difference between a "good stat" for a class and a "bad stat" can be  extreme. Some casters want Haste but not Crit; hunters want Armor  Penetration but not Haste. There are other overarching issues, as well,  such as Intellect not being very exciting for casters despite it being a  core stat -- and these are just a few examples.
Our ultimate  goal is make gear a more interesting (and less confusing) choice by  making each stat valuable to more players. While the reasoning behind  some of the following changes may be clear, we understand that you may  have questions about some of the less obvious alterations, and we'll do  our best to answer any questions you may have here on the forums.
What You’ll See on Gear
Stamina  - Because of the way we will be assigning Strength, Agility, and  Intellect, non-plate wearers will end up with more Stamina than before.  Health pools will be much closer between plate-wearers and other  classes.
Spirit - Come Cataclysm, this stat should only be  found on healing gear. Non-healing casters will have other systems in  place to regenerate mana, and we are designing special solutions for  Elemental shaman and Balance druids who often share gear with healers  (more on this below). Raid buffs that currently boost Spirit (such as  Blessing of Kings) will only boost the primary stats of Stamina,  Strength, Agility, and Intellect. We are also likely changing the  five-second rule and other quirks of the current regen system.
Intellect  - Intellect will now grant Spell Power (more on this below).  Intellect will also provide less mana than it currently does.
Haste  - Haste will become more attractive for melee classes by allowing  them to recover resources such as energy and runes more quickly. Our  intention is for Haste to let you "do stuff" more often.
Block  Rating - Block is being redesigned to scale better. Blocked attacks  will simply hit for 30% less damage. Block rating will improve your  chance to block, though overall block chances will be lower than they  are today.
Parry - Parry no longer provides 100% avoidance  and no longer speeds up attacks. Instead, when you parry an attack, it  and the next attack will each hit for 50% damage (assuming they hit at  all). In other words, Dodge is a chance to avoid 100% of the damage from  one attack, Parry is a chance to avoid 50% of the damage from two  attacks, and Block is a chance to avoid 30% of the damage from one  attack.
Mastery - This is a new stat that will allow  players to become better at whatever makes their chosen talent tree cool  or unique. It's directly tied to talents, so what you gain from  improving this stat is entirely dependent upon your class and the talent  specialization you choose. We’ll talk more about specific Mastery  benefits in the future.
Armor - The way Armor mitigates  damage is not changing, but the Armor stat has been rebalanced to mirror  changes to the armor curve in Cataclysm. As a result, bonus Armor will  go down slightly overall. We are also changing the mitigation difference  among armor types so that plate doesn’t offer so much more protection  than mail, leather, and cloth.
Resilience - This will only  affect damage done by players and critical damage done by players. It  will not impact crit chance, mana drains, or other such effects.
Strength,  Agility, Hit Rating, Expertise, and Critical Strike Rating - These  will all still appear on gear as well. Aside from situations mentioned  elsewhere in this list, in general these will function similarly to how  they do now, though the details -- such as how much Hit Rating you might  need to effectively combat high-level creatures (more on this below) --  are likely to change.
Being Removed from Items
Attack Power -  This stat will no longer be present on most items as a flat value,  though it will still show up on some process. Strength and Agility,  which will be present on items, will grant the appropriate amount of  Attack Power (generally 2 Attack Power per point of Strength or Agility)  depending upon which stat a particular class favors. Agility may  provide less Crit than it currently does.
Spell Power -  Spell Power is another stat that you'll no longer see present on most  items. Instead, as mentioned above, Intellect will grant Spell Power.  One exception is that caster weapons will still have Spell Power. This  allows us to make weapons proportionately more powerful for casters in  the same way they are for melee classes.
Armor Penetration -  This stat will no longer be present on items. Armor Penetration will  still exist in talents and abilities.
Shield Block Value -  This stat will no longer be present on items, since the amount blocked  is always proportional to the amount of damage done. Talents and other  effects might still modify the damage-reduction percentage from 30%,  however.
Going  Away Completely
MP5 - This stat will be removed  from the game completely. Holy paladins and Restoration shaman will be  redesigned to benefit from Spirit.
Defense - Defense is  being removed from the game entirely. Tanking classes should expect to  become uncrittable versus creatures just by shifting into Defensive  Stance, Frost Presence, Bear Form, or by using Righteous Fury.
Spell  Ranks - Spell ranks will cease to exist. All spells will have one  rank and will scale appropriately with level. The levels at which you  can learn certain spells are being changed in order to fill in some of  the gaps, and we will be introducing some new spells to learn along the  way as well.
Weapon Skill - This stat will be removed from  the game completely. Classes will start with all the weapon skills they  need to know and will not need to improve them.
What Else You Should Know
Combat  ratings - All ratings will be much harder to "cap out" at maximum  gear levels. Ratings will be steeper in Cataclysm, and creatures in  later tiers of content will be harder to hit or crit, similar to how  level-83 mobs are harder to hit or crit than level-80 mobs.
Reforging  - While these changes will go a long way to making a wider variety  of stats more attractive, we understand that sometimes you simply don’t  want more Hit Rating on your gear or you’d rather have more Haste than  more Crit. In Cataclysm, we are going to give players a way to replace  stats on gear as part of the existing profession system. As a general  rule of thumb, you’ll be able to convert one stat to 50% of another  stat. While some conversions (like converting Stamina to Strength) won’t  be permitted, the goal is to let you customize your gear more.
Gems  - We are changing the gem colors of a few stats as a result of  these adjustments. For example, Hit is likely to be blue instead of  yellow. We'll have more details on this in the future. 
Changes to Existing Gear
As  with previous expansions, we plan to roll out these changes and modify  all existing gear shortly before Cataclysm launches, though it’s still  too early to say exactly when. For the most part, the gear you have will  still be good for you, though there will be exceptions, such as  warriors using leather and mail armor.
If you are a tank (druids  excepted), expect to see:
- No more Defense on gear. Existing Defense becomes Dodge,  Parry, or Block Rating.
 - No more Block Value on gear. Existing  Block Value becomes Block Rating.
 - You’ll have as much Stamina as  you’re used to, though you may notice your tanking plate has a bit less  Stamina than a comparable piece of DPS plate, since we tend to take the  gem budget out of your most attractive stat.
 - Bonus Armor on  gear will go down slightly.
 
If you are a melee DPS class,  druid tank, or hunter, expect to see:
- A lot more Stamina. Bear-form Stamina scaling  will be lowered as a result.
 - Strength if you wear plate. Agility  if you wear mail or leather.
 - Existing Attack Power becomes  Agility and Stamina.Armor Penetration becomes Haste or Crit.
 - No  Intellect on melee gear. Hunters won’t need Intellect since they will no  longer use mana. Shaman and Retribution paladins will get mana and  spell damage in other ways.
 
If you are a DPS caster, expect  to see:
- A lot  more Stamina.
 - All of your Spell Power converted to Intellect and  Stamina.
 - No Spirit. You won’t miss Spirit, though, because you  won’t need it for DPS or mana regen.
 
If you are a healer,  expect to see:
- A  lot more Stamina.
 - All of your Spell Power converted to  Intellect and Stamina.
 - Spirit instead of MP5. You’ll probably be  happy with Spirit, though, because mana regen is going to matter more  than it does currently. Healing paladins and shaman will benefit more  from Spirit than they do currently.
 
If you are a Balance  druid or Elemental shaman:
- You will still share gear with Restoration druids and shaman.
 - Your  gear will have Spirit on it. It won’t have Hit on it.
 - You will  have a talent that converts Spirit to Hit. We will adjust talents  accordingly so that you want about as much Spirit as, say, a warlock  wants Hit.
 - Hit on rings and other such gear will still benefit  you.
 - Raid buffs will no longer boost Spirit, so you shouldn’t  find yourself unexpectedly over the Hit cap because of buffs.
 
Many  lower-level items with nonsensical combinations of stats, such as  Agility and Spirit, will be changed. We're also updating quest rewards,  trade skills, and loot drops to support better itemization for class  builds that weren't widely available or used prior to The Burning  Crusade (such as Balance druids).
We're aware this is a lot of  information to take in, but this is still only a piece of the larger  picture, and many of these changes rely on integration with other  systems we haven't yet discussed in detail. In the weeks and months  ahead, we'll continue to tell you more about these changes, along with  all of the new and exciting features we have planned for Cataclysm.