The combat ratings for hit and expertise are as follows:
Spell Hit Rating: 1% = 102.446 Hit Rating
Physical Hit Rating: 1% = 120.109 Hit Rating
Expertise Rating: 1% = 120.109 Expertise Rating
The caps for these stats are:
Physical Hit - Special or "Yellow": 8%
Spell Hit: 17%
Dual Wield - Auto Attack or "White": 27%
Up to the 8% special hit cap and the 6.5% expertise cap, physical hit rating and expertise are identical. 120 rating will increase your chance to land an attack by 1%. There are two exceptions to this rule that we are currently aware of: 1.) DK ghouls benefit from the DK's hit rating, but not expertise. This causes physical hit to be weighted higher than expertise for DKs. 2.) Ret Paladins have a couple attacks that are ranged physical attacks which use melee hit rating, but cannot be dodged or parried. This causes physical hit rating to be weighted higher than expertise for that class as well.
If spell hit rating also factors in for a class, a separate weight for spell hit should be provided. The game treats physical and spell hit rating differently, so we will too.
For an example of someone who understands this and created stat weights well, check out ieatpaperbag's posts for Assassination and Combat Rogues on the EJ forums. I will use Assassination Rogue stat weights (at the time of this post) as an example:
ieatpaperbag lists the stat weights like this:
Yellow Hit Rating = 1.75
Spell Hit Rating = 1.4
White Hit Rating = 0.75
Expertise Rating = 1.1
We can re-write these stat weights in the following manner:
Physical Hit Rating: 1.1 up to 8% cap, 0.75 up to 27% cap
Expertise Rating: 1.1 up to cap
Spell Hit Rating: 0.65 up to cap
Mr. Robot will always separate physical and spell hit ratings. In this example, up to the 8% cap, the total stat weight for hit rating is the physical + spell stat weight = 1.75. After 8%, the value is 0.75 + 0.65 = 1.4, and after 27% the value is simply 0.75.