PRIORITY STATS FOR A DPS DK
1 Strength = 2 Attack Power
(AP also scales into Spell Power for a DK which includes diseases and non-weapon attacks such as Death Coils. AP also increases the damage of Dancing Rune Weapon and Summon Gargoyle.)
62.5 Agility = 1% Critical Strike Chance
(Only take Agility if it comes on gear and the total AP bonus combined with other more useful stats on the item outweigh a similar item with Strength. Similarly, only enchant or gem an item with Agility if there is no better option available.)
1 Agility = 2 Armor Rating
(If specced for Bladed Armor 5/5 in the Blood tree, Agility will indirectly increase your Attack Power by 1/18 or, in other words, 18 Agility will increase your Attack Power by 1 Point - good to know, but still not enough to stack for Agility.)
45.91 Critical Strike Rating = 1% Critical Strike Chance
(In nearly every situation taking Critical Strike Rating over Agility is a better option simply because a DK gets more "bang for their buck". Basically, Critical Strike Rating grants a DK a higher Critical Strike Chance than Agility for the same item budget. Critical Strike Rating also affects both Melee and Spell Critical chance while Agility does not. All DK abilities except Icy Touch, Howling Blast and Death Coil use Melee Critical Strike Chance. DK's also have a passive critical damage increase called Runic Focus that grants double damage on Melee and Spell Critical Strikes)
32.79 Hit Rating = 1% Melee Hit Chance
(The required chance to hit for all level 80 2H Weapon Wielding classes against boss-level mobs is 8% or 262.32 Hit Rating. The required Hit Chance for like-level mobs while leveling or in PVP is 5% or 163.95 Hit Rating. Hit Rating affects every DPS attack a DK employs with the exception of disease ticks, which is important to know because a 1% increase in Hit Chance will produce a greater than 1% increase in overall damage. The value of Hit Rating also increases the value of Critical Strike Rating for DK spells.)
26.23 Spell Hit Rating = 1% Spell Hit Chance
(For all intents and purposes, Spell Hit Rating and Melee Hit Rating are combined into the "all-encompasing" Hit Rating, but I will address each of them exclusively. The required chance to hit with spells for all level 80 2H Weapon Wielding DKs against boss-level mobs is 17% or 445.91 Spell Hit Rating. Remember, only Icy Touch, Howling Blast and Death Coil use Spell Hit to check if they have a chance to miss. Speccing into Virulence 3/3 in the Unholy tree will lower a DKs chance to hit with spells by 3% or an overall Hit Rating of 367.22 and is lowered another 3% or an overall Hit Rating of 288.53 if a Boomkin (Imp. Faerie Fire) or Shadow Priest (Misery) is present in the raid. Having a Draenei in party (for Alliance) will also increase chance to hit with spells and attacks by 1%.)
8.2 Expertise Rating = 1 Expertise Skill
(1 Expertise Skill decreases the chance that melee attacks made by the player will be dodged or parried by 0.25%. 32.79 Expertise Rating is equal to 1% less chance your attacks will be dodged or parried. The required Expertise cap to prevent Dodges from boss-like mobs is 214 Expertise Rating or 26 Expertise Skill, which roughly equals 6.5%. The cap to prevent parries is somewhere around 12-15% or 492 Expertise Rating, but is irrelevant for melee DPS as they should be attacking bosses from behind in every situation. Every DK talent tree has talents that increase their Expertise Skill such as Veteran of the Third War 3/3 in the Blood tree, Tundra Stalker 5/5 in the Frost tree and Rage of Rivendare 5/5 in the Unholy tree.)
12.31 Armor Penetration = 1% of Ignored Armor on Target
(All diseases, Icy Touch, Howling Blast, Scourge Strike and Death Coil receive no damage bonus from ArP, which makes ArP more useful for specs focusing mainly on physical damage such as Blood.)
25.21 Haste Rating = 1% Haste
(Haste directly increases the amount of white swings to a DK's DPS, which doesn't produce an overall desirable effect because DK's generate very little of their DPS from white damage. Almost every DK attack is an instant attack outside of weapon swings, which means Haste actually does very little to speed anything up in the DK's arsenal. Haste does affect a DK's GCD (Global Cool-Down) giving them a "wider margin for error", but doesn't add much DPS potential overall. Haste may be more useful for dual-wielding DKs at some point, but should probably be passed over in favor of more useful stats.)
SECONDARY OR NON-APPLICABLE STATS FOR A DPS DK
Intellect technically increases a DK's Spell Critical Strike Chance, but DK's gain more through basic Critical Strike Rating. DKs should never have an item with this stat or chant/gem for it.
Spirit increases a DK's health regen rate, but outside of the occasional Priest buff a DK should never have this stat on an item or chant/gem for it.
Stamina gives 10 Health Points for every 1 Stamina Rating. Nearly all items a DK will use will have a generous amount of Stamina on it, therefore a DK should never chant/gem for this stat other than the special case of a few gems that include other stats, in addition to Stamina, for a DK.
SPELL POWER IS OF ABSOLUTELY NO USE TO A DK as all of a DK's spells scale with their Attack Power. In no situation should a DK ever have any gear whatsoever with Spell Power.
SO WHAT DOES IT MEAN?
At this point you might be asking yourself what all of this information means or maybe just want a simple breakdown of what to do to start pumping out some crazy good DK DPS. Here's my opinion for whatever it's worth...you can take it or leave it.
In general, I would prioritize, or give weight, to DK stats as follows:
BLOOD
WEAPON DPS > STRENGTH > ArP > MELEE HIT > EXPERTISE > CRIT. > HASTE > AGILITY > AP > SPELL HIT > ARMOR
FROST
WEAPON DPS > STRENGTH > ArP > MELEE HIT > EXPERTISE > CRIT. > HASTE > AGILITY > AP > SPELL HIT > ARMOR
UNHOLY
WEAPON DPS > STRENGTH > MELEE HIT > EXPERTISE > HASTE > CRIT. > AGILITY > ArP > AP > SPELL HIT > ARMOR
As a level 80 DPS DK I would focus on getting melee hit capped (262.32 or 8%) as my first priority. Getting expertise capped (26 or 6.5%) is going to be extremely difficult for a fresh 80, so I would cap out the tree specific expertise telents mentioned above and start looking for expertise gear as you begin raiding and running heroics (this stat, unfortunately, is found much more frequently on higher level gear). I would also go ahead and spec 3/3 into Virulence in the Unholy tree which will help you with the spell hit cap. If you know you will have a Boomkin or Shadow Priest in your raid and you spec into Virulence you will only need (26.21) in addition to your melee hit cap (262.32) to be both melee and spell hit capped (288.53), which is very doable with gear, chants and gems at level 80. Having a Draenei in party (for Alliance) will also increase your chance to hit with spells and attacks by 1%.
In most cases, gemming for strength (even to the point of ignoring socket bonuses) and chanting for AP (best option available) is the best course of action, but do not underestimate the effect the melee hit cap and expertise cap will have on your overall dps production. At some point, as a Blood DK, if your AP (raid-buffed) consistently stays above the 6k+ mark, gemming and chanting for ArP might be more beneficial for higher DPS production (this most likely will not be true for Frost and Unholy). Haste, Crit., Agility and AP should almost never be gemmed or chanted on gear unless their is no better option available (ex. Greater Speed on Cloak, Icescale Leg Armor on Leggings, Crusher on Gloves etc. etc.)
Notice: I did not write this guide.