I read up a bit on ElitisJerk.com to figure out exactly about what you were asking. I got a great unholy spec and a lot of answers.

Haste isn't all that good because it mainly works for just our melee swings. Melee is a good chunk of dps but our talented strikes and diseases are our real bread and butter and are unaffected by haste.

Armor penetration is a meh stat because, like haste it's for white swings. Diseases will increase our damage is unaffected by Armor penetration.

Strength is our most important stat because it gives us parry, ap, and if you're unholy and grabbed impurity, it directly increases the effect of that talent, which in turn increases your damage from diseases, abilities, and spells.

Critical Strike Rating also isn't all the important due to the fact that our dots do a lot of damage and crit doesn't work for them. It's not an ignorable stat you want some crit, it's just not all that good of a stat.

Agility is a good alternative to critical strike rating due to the fact that it also gives us dodge for tanking, and crit as well.

Crit affects our melee swings, strikes, and dc. So it's good to have but not super important.

Head to ElitistJerk.com, might be jerks, I always mix it up. They have an excellent article on DK dps for all specs + glyphs and hit rotations. My dps shot up after tweaking one of their unholy specs to my liking and I love it.

~Droctognapus, 80 Death Knight, Aegwynn