Blizzard uses a very basic averaging scheme to determine your over all ilvl.
This site I used today to make sure my Hunter really was at 468 ilvl (per the character window) I used it and sure enough I am at ilvl of 468.9333 Trying to get to 470 to start the second set of MOP Raids on my hunter.
This is the site, its not WOW related at all.
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www.calculatorpro.com/ca ··· culator/You will have to enter in each items ilvl seperated by a comma.
Like this.
This is currently my ilvls for my Warrior not hunter.
483,489,489,489,483,483,483,483,476,496,483,489,471,476,491,471
I average them and get 483.4375 which is very close to the Character pages reporting of 484 ilvl. (Counting my Off Hand)
Now since he is a Warrior I then exclude the offhand weapon. I get 484.26666666667 which is spot on what I have in ilvl.
So exclude your offhand for dual wielders. Because other classes (Like DK's and hunters) have to be balanced around the same averaging scheme.
Average your Ilvl by main hand only. Also remember there are 15 gear slots with stats, not counting offhand.
With that said as far as stat goes. I will link you the Wiki about classes and what there stats are.
www.wowwiki.com/Class
Basically its like this with Stats.
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www.wowwiki.com/AttributeStrength,Intellect,Agility,Spirit and of coarse Stamina are all primary Stats.
Hit,Expertise,Crit,Haste and Mastery are secondary stats.
You will see these all under your Characters profile accessed by hitting "C" by the default keybind.
Then lower right corner there is a yellow arrow that opens the attributes tab. Under this tab, is General-Attributes-Melee-Ranged-Spell-Defense
General is where you will find the average ilvl of all your equipment.
It will factor that which is on the toon and that which is in your bags. Like this 480/484 or similar. Worn/Bags is the scheme.
"Attributes" are the main stats to all toons. But the Melee/Ranged/Spell is what you will pay attention to depending on your class.
Example is Warlock/Mage will look to Spell, Hunters will look to Ranged and Melee is for Warriors/DK/Paladin etc etc.
The Defense Tab is a bit special for that of Tanks. Because they need weigh not only Melee Stats but also Defensive stats.