Expertise: This statistic is actually relatively useful to tanks, who may want to consider getting a good amount to prevent parries and dodges. It takes approximately 8.2 expertise rating to equal 1 expertise at 80. You'll need about 26 expertise to prevent dodges, which is all a DPS should need since mobs can't parry from behind, or about 60 expertise to prevent parries, which would be nice for tanks, but shouldn't be taken at the expense of too many mitigation or avoidance stats.
This statistic loses some of its usefulness when we remember that it works only with physical attacks, and spells can't be dodged or parried, Death Knight specs that focus more on spell-like damage may find they'll do better stacking more strength instead of expertise. Still, physical damage specs like Blood may also find that getting that dodge cap in expertise, which shouldn't be too painful thanks to talents, may be just what they need to get their DPS to the next level.
Armor Penetration: Like expertise before it, the problem with this statistic is that it doesn't work on spells, or rather, doesn't work on non-physical damage, which bypasses armor automatically. Thus, it's difficult to justify taking this, especially armor penetration was nerf in Wrath from straight values to a rating that gives a percentage rating that is applied only after any armor debuffs.
Still, once again, physical damage heavy specs (That is, Blood) may find some limited usefulness from this statistic, but any spec that does most of their damage via magical or elemental means (that is, pretty much any permutation of Unholy or Frost) would probably be better advised to stack more strength, attack power, and critical strike rating.
Haste: The problem with haste for Death Knights is that pretty much all of our attacks are instant outside of weapon swings, meaning haste has very little to actually speed up. Even speeding up our global cooldown is a bit iffy. Since haste doesn't actually speed up the cooldown of our runes, we may not even have anything to cast with that extra time. Tri-spec Dual wielders, who rely on fast swings for more procs from Killing Machine and more damage from Necrosis, may end up getting some use out of haste, but most Death Knights should probably avoid it in favor of other stats.
Agility: Agility adds melee critical strike rating, dodge, and armor for a Death Knight. The latter two actually make it a decent choice for a Death Knight tank, especially an Unholy Death Knight tank looking to keep Bone Shield up longer. For DPS, though, you'll want to grab critical strike rating for your critical strike needs since you want spell critical strike too.
Intellect: Technically, this statistic does give you spell critical strike percentage, but you'd be better off getting raw critical strike rating, since that applies to melee critical strike as well. Avoid this statistic.
Spirit: Technically, you do get health regeneration from this statistic, but honestly, even non-Blood Death Knights rarely need more than Death Strike and a couple bandages to stay topped off in most situations. Avoid this stat on your armor.
Block Value and Block Rating: Both of these statistics deal with shield usage and blocking, which don't apply to us. You may find a piece of tank gear every once in a while which is worth taking in spite of block value because of how good it is otherwise, but the block statistics themselves do nothing for us.
Spellpower: Despite our use of spells, we don't need this at all. All of our spells scale with attack power instead of spell power. Unless you genuinely have no other option for the slot (and you probably always will have one, if only thanks to our starting zone quest armor), don't wear armor with spell power.
The Run Down and the Wrap-up
So in essence, the too long, didn't read version of the above goes something like this:
- DPS will want hit rating to the 293 cap, strength, critical strike rating, and maybe some expertise, in that general order.
- Tanks will want defense rating to the cap, stamina, armor, parry rating, dodge rating or agility, strength, and a solid amount of expertise and hit rating, also in that general order, with some allowance for more strength or hit rating if more threat generation proves needful.
- Intellect, spirit, block value, block rating, and spellpower are worthless to us. Don't take them.