With the release of Patch 2.0.1, a new mechanic has being introduced: Combat Ratings. These Ratings are being used for any combat stat that previously was percentage-based such as: critical strike chance, hit chance, dodge chance and defense skill. Combat ratings are only used with effects generated by items and do not apply to effects that are generated by spells and talents which will continue to work the same.
Combat Rating Types
The following combat ratings are currently in use: weapon skill, defense, dodge, parry, block, hit chance, spell hit chance, critical strike chance, spell critical strike chance, resilience, haste, and spell haste.
Combat Skills
Unlike fixed percentages such as 2% critical strike chance, combat ratings diminish in potency as your character increases in level. 2% crit is the same at every level, while 28 critical strike rating grants 4% crit at level 34, 2% crit at level 60, and 1.27% crit at level 70. This allows 
[Blizzard] the ability to create and add new and better items to the world without eventually reaching a point where every character has a 100% chance to critically strike.
Below are the conversion for level 60 and 70 combat skills:
| Rating | Effect | Requires | |||
|---|---|---|---|---|---|
| Level 60 | Level 70 |
Template:GenericRow1 Template:GenericRow2 Template:GenericRow1 Template:GenericRow2 Template:GenericRow1 Template:GenericRow2 Template:GenericRow1 | |||
Defense Skills
The impact on the defense skill and weapon skill systems is slightly more complicated. Many people do not realize these skills actually grant percentage-based benefits already. For example, every 25 points of defense skill grants a 1% dodge chance, 1% parry chance, 1% block chance, 1% increased chance to be missed and 1% decreased chance to be critically hit by physical attacks. Weapon skills have a similar effect for the attacker. Items will now grant skill rating rather than skill directly, and that will convert to an actual skill increase.
Weapon Skill now does the following:
- Weapon skill will no longer reduce the percentage damage lost due to glancing. - The player will gain 0.1% to their critical strike rating per weapon skill against monsters above their level.
Below are the conversions for level 60 and 70 defense skills:
| Rating | Effect | Requires | |||
|---|---|---|---|---|---|
| Level 60 | Level 70 |
Template:GenericRow1 Template:GenericRow2 Template:GenericRow1 Template:GenericRow2 | |||
Round-off problems
When Blizzard made the conversion to 2.0.1, all existing items with the old +crit%, +defense, etc. values were converted to their equivalent values in the new Ratings system scaled for a level 60 character. For example, an item that previously gave +10 Defense Skill now gives +15 Defense Rating.
One problem, though, was what to do when the new rating would be a fractional value. A +5 Defense Skill item in the old system should give +7.5 Defense Rating in the new system, but fractional values are not allowed. The Rating has to be a whole number. Should this item's new Defense Rating be +7, or +8?
Ultimately, Blizzard decided to round all these fractions down to the nearest whole number. A +5 Defense Skill item became a +7 Defense Rating item. If the character had a lot of items with an odd-numbered amount of +Defense Skill in the old system, he would find himself with a significantly lower Defense Skill in 2.0.1 than he had with exactly the same gear in 1.12.1.
Worse, some +Weapon Skill items received a reduced amount of +Weapon Skill Rating, even though their original +Weapon Skill amounts were even numbers. For example, the 
[Sceptre of Smiting] gave +2 Weapon Skill with maces in 1.12.1, but only gives +4 to Mace Skill Rating in 2.0.1, even though from the above charts it should have a Mace Skill Rating of +5.
Resilience
Resilience is a special new rating which was created to reduce the effects of critical hits against your character. It has two components; it reduces the chance you will be critically hit by x%, and it reduces the damage dealt to you by critical hits by 2x%. x is the percentage resilience granted by a given resilience rating.
| Rating | Effect | Requires | |||
|---|---|---|---|---|---|
| Level 60 | Level 70 |
Template:GenericRow1 | |||
Levelling
Each time you go up a level, the amount of rating needed to get the same benefit will increase. An example of the scaling involved would be the current implementation of Agility which has always worked this way in the live game, requiring more agility for the same critical strike chance as you go up in level.
Combat Ratings Formula
The following formulas give the amount of rating required to achieve a 1% bonus. They can be derived from two sources. They are from the values stored in the client, and from extensive testing done by the author of the Rating Buster addon, both of which gives the same values and the formulas below used to model the combat system gives an error margin of around 10^-7, which is within the margin of error for 32-bit floating point precision.
Level 1 to 60 (level 1-9 uses level=10):
base * (level - 8) / 52
Level 60 to 70:
base * 82 / (262 - 3 * level)
Level 70 to 100:
base * (level + 12) / 52
Where base is the rating value at level 60:
Weapon skill: 2.5 Haste: 10 Hit (melee): 10 Hit (spells): 8 Crit (melee and spells): 14 Defense: 1.5 Dodge: 12 Parry: 20 Block: 5 Resilience: 25
Source
Combat Rating System Explanation | 10/16/2006 06:06:07 PM PDT by Eyonix
Combat Ratings: Level 70 Conversions | 10/20/2006 11:29:55 AM PDT by Eyonix
Combat Ratings: Rating Buster AddOn and its data points | 11/05/2006 04:23 PM GMT+1 by Whitetooth
Blizzard BETA Combat Rating System Notes | 12/09/2006 02:46 PM GMT by Drysc. Link posted by Ych (after finding the above links broken/missing/otherwise faulty)
Blizzard Combat Rating Conversion Factor at 70 | 12/09/2006 15:15 PM GMT by Eyonix