Burning Crusade items will no more contain normal direct statistics like "1% spell crit" or "1% parry". At some point a border would be passed, where items become simply too powerful. Of course weapons and armor for level 70 players have to be more advanced than the current versions. But Blizzard can't create for example a cloth belt with 3% or 4% spell crit rating just to beat the current Naxxramas items. The balancing and the pvp fights would be totally destroyed.

Therefore Blizzard introduces with Burning Crusade the "rating" system - a kind of brake for your current equipment. Take the Frostfire Robe (T3 mage set) as example. This possesses among other things 1% spell crit and 1% spell hit. In Burning Crusade change those to "14 spell critical rating" and "8 spell hit rating". These values (14 and 8 ) correspond to their counterparts specified above (1% crit and 1% hit). Now the important point: This valuation refers ONLY to level 60. As soon as you level up (61-70), the conversion sinks and your 14 spell critical rating corresponds no longer to the full 1% crit. As you can see your items will decrease in quality while your leveling up to 70 (the exact conversion factor of level 70 is yet not known).

Blizzards developers secure this way that they can still design cool items without having to worry too much about balancing and damageoutput. In addition they can tweak the increases much finer (for example 1/2 spell hit chance).

Here is a table for converting all relevant item characteristics into the new Burning Crusade system (remember: the values apply ONLY to level of 60 characters - a level 70 character has a much lower real crit percentage with the same amount of rating points):

1% spell crit - 14 spell critical rating
1% spell hit - 8 spell hit rating
1% crit strike - 14 critical strike rating
1% chance to hit - 10 hit rating
1% parry - 20 parry rating
1% block - 5 block rating
1% dodge - 12 dodge rating
07 defense - 10 defense rating
09 defense - 13 defense rating
11 defense - 16 defense rating