Taken from this post by Ghostcrawler. Here is a summary
As a side note: JUST GET RID OF PVP GEAR. It's stupid.
Spell resistance is gone. There are no buffs that improve it and there shouldn’t be much, if any, spell resist gear left. We always thought the system was hard to understand and we weren’t getting much gameplay out of it. Now taking a step back, we can imagine how to develop a game where you’d want various forms of resist gear for certain situations and opponents. Resist gear could potentially be interesting, but it isn’t currently in World of Warcraft -- the game has just been moving away from that sort of thing for years.
In the absence of spell resistance, there is no need for spell penetration on gear, so we’ll remove it as well.
Hit and Expertise
We still think having stats that can be capped is a good game design. Rather than focusing solely on stacking your best stat, you have to decide how valuable it is to hit your target before you go back to stacking your best stat. However, we are making some changes.
Hit and spell hit will no longer be separate stats. The hit stat negates melee miss and spell miss.
Expertise will negate dodge and spell miss, then parry.
Expertise will be listed as a percentage, just like hit, instead of having an intermediary stat.
We are normalizing hit with expertise, so that 1% of each stat will require the same amount of rating.
We are normalizing melee and spell hit, so that spell miss is equal to melee miss plus dodge.
Against an equal level creature: 6% spell miss, 3% melee miss, 3% dodge, 3% parry (from the front only), 3% block (from the front only).
Against a +1 level creature: 9% spell miss, 4.5% melee miss, 4.5% dodge, 4.5% parry (from the front only), 4.5% Block (from the front only).
Against a +2 level creature: 12% spell miss, 6% melee miss, 6% dodge, 6% parry (from the front only), 6% Block (from the front only).
Against a +3/boss level creature: 15% spell miss, 7.5% melee miss, 7.5% dodge, 7.5% parry (from the front only), 7.5% block (from the front only).
Ranged attacks will be able to be dodged. Hunters will benefit from expertise and will have it on their gear, which will also allow hunters and Enhancement shaman to share gear more easily.
The chance to block will be handled by a separate combat roll for each attack that is not avoided. In other words, we first determine if an attack misses, or is dodged or parried. If it is not, then the attack has a chance to be blocked.
This gives block a consistent value, regardless of avoidance. Currently block becomes more valuable the more you have.
Block will also have diminishing returns, much like dodge and parry. This doesn’t mean that the value of block will go down as you get more block. It means that it won’t go up by as much when you get more block.
We don’t expect Protection warriors or paladins to get “block capped” other than during temporary effects, such as mastery procs on trinkets. Block tanks will be balanced around this change. Our intent is to make playing block tanks more fun, not to nerf them.
Also notice how Shield Block and Shield of the Righteous have changed in Mists.
All spells and abilities will crit for double damage, baseline. There are a few exceptions where crits can get larger, but the default is x 2.0 for everyone.
This means that Enhancement shaman spells and rogue poisons will crit for double damage. Rogue poisons will also use the melee hit chance.
We are renaming this stat to “Defense (PvP)” or possibly “PvP Defense.” All players will have 30% base Defense, the same way all characters have some base Stamina.
PvP gear will have Defense on it, as well as a new stat, “Power (PvP).” Power increases the damage you do to other players as well as the healing you do to other players in PvP situations.
If you have a lot of Power, you’ll do more damage to other players, but they likely have Defense as well. If you fight players in lots of PvE gear, they’ll take more damage. Likewise, a player in PvE gear won’t have enough Power to effectively penetrate your Defense.
The names PvP Power and PvP Defense may not be final, but we’re leaning towards going with stat names that are obviously PvP-related, rather than “fluffier” names that might not be as easy to grasp. We want it to be clear to players that neither Power nor Defense have any relevance when fighting creatures, such as in dungeons or raids.
PvP gear will be lower in item level than PvE gear of an equivalent tier, however the Power and Defense stats will make sure that PvP gear is more powerful in PvP (both offensively and defensively) than PvE gear. In our budgeting system, the PvP stats will be free rather than causing other stats, such as Strength or haste, to be smaller as a result of including Power or Defense.
The goal of this change is to make it easier for a PvP player to participate in PvE, or for a PvE player to get started in PvP. Currently, we feel it is too large a barrier to go from one to the other, and the result has been that we see more and more players choosing to focus exclusively on only PvP or PvE. In earlier expansions, it was more feasible to use PvE gear in Arenas or Battlegrounds until you acquired the more useful PvP gear. The same was true of being able to use your PvP gear in a dungeon or raid until you acquired something better. In Cataclysm, stepping into PvP with no PvP gear would result in a player being so ineffective that it was difficult to even make progress towards acquiring PvP gear.
For the higher-end of PvP or PvE (say Gladiators or heroic raiders), we believe those players will still gravitate towards the dedicated PvP or PvE gear. It is the players who are working towards those two end games that will benefit more from some cross over.