There was a post recently on the new stat system. I've included it here:

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.

I think there are some interesting points in there. Particularly those about Mastery and Reforging. I'm not surprised to see Spirit reworked in such a way and quite frankly, I think it will help simplify the loot tables. One consequence that I see from all of this is that we will now be able to wear mostly leather gear, since we won't mind (and in fact will need) the spirit it provides. Moreover, Reforging will allow us to convert any extra spirit we might pick up into Int or Mastery.

I will say, however, that this system doesn't seem inherently less complex, which seems to be a driving goal. Mastery is vague to say the least, and I wonder how it will compare to Intellect (the new SP) or Crit or Haste. After all, if Mastery makes a spell hit harder, isn't it basically the same as Spell Power? And then there is this:

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.

I do have a major concern with haste. Allowing melee classes to recover resources more quickly seems to provide yet another scaling method for them. As a spec that is already having trouble keeping up with the Jones's, I worry that additional scaling with melee will make them more desirable at higher levels of gear. Perhaps this is the point of Mastery (to provide extra scaling).

All told, it's a little to early to cast predictions or theorycraft any of this. I expect that although Blizzard wants to make gear choices interesting, we will find a way to reduce the choice to an equation, despite the introduction of new variables. I still anticipate being able to find BiS lists and I still anticipate gearing guides. Only time will tell...