In Patch 2.3 Blizzard are introducing a new Skill and Rating, Expertise, which will replace the current Weapon Skill/Rating and with it introduce a new and understandable mechanic.
The Mechanics
Expertise reduces the dodge and parry chance of the target by 0.25% per point of skill.
Nice and simple.
Blizzard have said that the Rating / Skill ratio will be the same for Expertise as it is for Weapon Skill.
This means that 1 Expertise Skill = 3.9 Expertise Rating at level 70.
So now things are easy to understand, the mechanics of Weapon Skill have always been subject to conjecture and the analysis of statistics generated from combat. Now we have the mechanic of Expertise stated by Blizzard and it could not be simpler.
How does this affect us?
We lose the hit rating and crit rating bonus from Weapon Skill.
Hit cap for Combat Rogues with Expertise on mobs at +3 lvl is now 365.
We gain a much higher reduction in dodge and parry chance of the target.
Weapon Skill has been shown not to actually reduce parry chance (the mechanic has been broken for sometime in the game). More on Dodge chance later.
Expertise affects all attacks and not just white dmg. Dodged specials cost 25% energy, generate no CP and do no dmg.
Expertise does not have a diminishing returns curve after the first 5 skill points. It scales linearly with each point of rating (3.9 Rating = 0.25% reduction) and is capped only by the mobs dodge/parry rating.
For mobs of equal level from behind: 5% dodge chance, can be negated by 5 * 3.9 * 4 Expertise Rating = 78 Rating or 20 Expertise Skill.
For mobs of lvl +3 from behind: 8.6% dodge chance, can be negated by 8.6 * 3.9 * 4 = 134.16 Expertise Rating or 34.4 Expertise Skill.
This means that Expertise will be of benefit in all forms of combat, PvE and PvP.
It is a general melee skill, this means that Tanks get the same benefit of dodge/parry reduction as other melee combat classes. Better threat generation and improved rage efficiency.
Expertise turns Dodged/Parried attacks into hits at a rate which offsets the loss of hit rating from Weapon Skill.
With 5 Expertise Skill you are worse off than when you had 5 Weapon Skill.
With 10 Expertise Skill you are better off then when you had 10 Weapon Skill and this gets better the more Expertise you have.
It is worth stacking Expertise Skill as long as you don't sacrifice hit for it.
The loss of 3.5% hit with 10 weapon skill for white dmg is made up for by the 2.5% reduction on dodge for all attacks in Raids and Expertise is very much a buff in normal PvE and PvP.
IMHO this makes things easier to work out as far as what stats you need and is a buff in the long term. Solo and PvP players are definitely better off.
Obviously some gear tweaking is needed, I would say around 270 hit rating with Precision + 10 Expertise Skill is the same as 250 Hit Rating + Precision and 10 Weapon Skill.