Overview:
Expertise is a new stat being introduced in 2.3 that is replacing weapon skill. It reduces the chance that your attacks will be dodged or parried. Unlike weapon skill, there is no "mace expertise" or "dagger expertise". Expertise is expertise, and it will affect any weapon you wield. 3.9 expertise rating will equal 1 expertise at level 70, and each point of expertise reduces your chance to be dodged and parried by 0.25%.

Weapon Skill:
All weapon skill increasing effects have been removed. In the case of racial traits, +5 weapon skill has been changed to a +1% crit chance with that weapon type. This includes melee racials, even though the patch notes fail to mention them. Gear and talents that formerly gave weapon skill now give expertise instead.

Rogue Talents:
Mace Specialization and Weapon Expertise have been changed. The following links are screenshots of their new tooltips.
http://www.reverierealms.com/macespec.jpg
http://www.reverierealms.com/weaponex.jpg

Expertise vs. Hit Rating:
Just like hit rating, expertise reduces the changes that your attack will "fail". Because you should normally be attacking from behind, the parry portion of expertise will not have any impact on your DPS. However, mobs can still dodge attacks from behind. The amount of impact expertise and hit have on your attack table are as follows:
15.8 hit rating = -1% miss
15.8 expertise rating = -1% dodge
Thus, hit and expertise are both equally beneficial. As long as you're not at the cap for either, 1 hit rating is equivalent to 1 expertise rating when making gear choices.

Caps:
Base miss chance when attacking a raid boss is 28% for a dual-wielding rogue. I'm not sure about dodge, but I'm going to assume that it starts with a 5% base chance. The two current theories on how weapon skill affects dodge chance when attacking a mob are 0.04% per point and 0.1% per point. The first would mean (assuming the base is 5%) a 5.6% dodge chance from a boss, and the second would result in a 6.5% dodge chance. So, caps for hit and expertise rating would be...
442 hit rating without talents or 363 hit rating with 5/5 Precision
78 expertise rating or 39 expertise rating with 2/2 Weapon Expertise if dodge chance is 5%
90 expertise rating or 51 expertise rating with 2/2 Weapon Expertise if dodge chance is 5.6%
102 expertise rating or 63 expertise rating with 2/2 Weapon Expertise if dodge chance is 6.5%

Why raise the hit cap again?
Some of you may be complaining that now you have another stat that you have to stack up to a cap before you can focus on AP, crit, and haste. Well, you're wrong. You don't have to stack expertise rating, just like you don't have to stack hit rating. You should stack them, though, because until you reach the cap, they're going to be the cheapest way to use your item points to increase your DPS. Adding expertise isn't gimping your possible DPS by making you have to stack yet another stat. It's increasing your potential DPS by allowing you to completely remove all of a boss's avoidance from the attack table and ensure that every attack you make will hit or crit.

Is this a buff or a nerf?
To those who had at least +5 weapon skill prior to 2.3 (meaning every Combat rogue), this is going to be a nerf at first. You're going to have to spend extra item points to make up for what you've lost. However, in the end, you'll be able to completely remove avoidance from your attack table, which you couldn't do before. Additionally, expertise is expertise. What that means is that you don't have to worry about getting dagger skill or sword skill; you can just get expertise. It also frees Blizzard up to put expertise on items other than weapons. Some of the new heroic badge rewards have expertise on them, for example.

Conclusion:
Expertise is an interesting new stat that opens up new possibilities for melee DPS classes. For the first time in WoW, you are able to ensure that every attack you make will land (whether it be a hit, a crit, or a glance).

Edit: Added expertise cap with 2/2 Weapon Expertise talent. (10/12/07)
Edit: Added additional dodge possibilities to the Caps section. (10/25/07)

Blah to this new skill.

Meh... sounds like something unachievable till WotLK.

Tagged for reading once I get to work.

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Q u o t e:
Meh... sounds like something unachievable till WotLK.

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Not really, With Vashj belt and Wep expertise alone you can get 16ish, and the cap is likely around 20-25 depending on the exact amount of dodge 73+ have.

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Q u o t e:


Not really, With Vashj belt and Wep expertise alone you can get 16ish, and the cap is likely around 20-25 depending on the exact amount of dodge 73+ have.

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The gnome suggested 102 Expertise rating to cap dodge.

You read the OP, didn't you?

So a buff to our potential, but a nerf to our reality?

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Q u o t e:
The gnome suggested 102 Expertise rating to cap dodge.

You read the OP, didn't you?

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I'm under the impression we're be getting 1 expertise point where we would have gotten 1 weapon skill in the past. So 6.5% dodge would be 26 expertise which would be 101.4 (102) expertise rating. However we're all going to have the 10 points of expertise from talents, so instead of 102 it will be 101.4 - 39 = 62.4 (63) rating that we will need to get on our own to cap on bosses. We get 10 of the 26 expertise we need, and have to get the other 16 from gear.

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Q u o t e:
Wep. Ex gives ~ 39 Expertise rating, 100D has 25 Rating, that's 64 right there.

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Thanks for pointing out that you wouldn't need the full 102 with Weapon Expertise. Somehow I overlooked that during my initial write-up. >.>

It's been added.

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Q u o t e:


Thanks for pointing out that you wouldn't need the full 102 with Weapon Expertise. Somehow I overlooked that during my initial write-up. >.>

It's been added.

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Wow way to credit Ippon because he barely beat me!

And you said 64, the cap is 63.

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Q u o t e:
The gnome suggested 102 Expertise rating to cap dodge.

You read the OP, didn't you?

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L2Convert nub.

http://www.reverierealms.com/macespec.jpg


Woot that just made mace spec even stronger in PvP.

Ah yes. The different between expertise and expertise rating. I'd better start getting used to differentiating between the two.

So Weapon Expertise will give straight Expertise, not rating, which will affect target dodge %.

So, to reiterate, with Weapon Expertise we'll have 10 expertise which is worth 39 rating and only need 62.4 (63) expertise rating from gear.

I think I got that right.

The benefit from expertise that you're all kind of missing I think is that it applies generally to all weapon types. For a level 70 rogue with daggers/swords/fists/maces skill already maxed it's not that big of a deal. But for a mid level hunter, for example, it's really nice. Now I can switch to whatever best weapon I come across without having to re-level my weapon skill for that specific weapon type.

reported for sticky.

This will be asked multiple times a day.

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Q u o t e:

Wow way to credit Ippon because he barely beat me!

And you said 64, the cap is 63.

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Actually, it was 102.7, not 102. I just rounded it down instead of up out of habit, since I'm used to truncating things when it comes to WoW. Technically, the cap should probably be 103. Either way, one point (probably) isn't going to kill you.

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Q u o t e:
So, to reiterate, with Weapon Expertise we'll have 10 expertise which is worth 39 rating and only need 62.4 (63) expertise rating from gear.

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If boss dodge really is 6.5%, which seems logical (and therefore may be wrong!)