Frost DK with some blood for healing. Blood is only good for botting with a lot of add's. Because the Heart Strike AoE and death coil proc's a lot more when HS hit 2 targets.
Unholy is good too but I guess with the GCD problem not viable. Best race is the Space People because the HoT is simply deadly and 1% hit is extra cream. After that I would say orc for pet damage and a little boost on axe expertise, but most significant difference you will find using the Space People.
I would rank:
Frost (with new patch)
Unholy >= Blood
For high spawn 80 grind:
Blood (with ghoul)
Frost [close second]
Unholy [distant 3rd]
61 Frost:
http://www.wowhead.com/talent#joZGMIxAbczc0cuzio:kV
80 Frost:
http://www.wowhead.com/talent#jfVhMhzZGMIxAbczc0cuzAo:kV [Death Rune Mastery ensures you will have Frost Runes to pull with Icy Touch, when your mowing down mobs at 4sec per kill]
Problem is you need 2 weapons, so I suggest stick with blood or unholy until level 61+ when the overpowered DK gear degrades and you get Obliterate.
You will be killing so fast that there will be no need for healing. Get up to Howling Blast, then go blood for Vendetta (don't get Rune Tap, Vendetta+Death Strike is enough). Edit the CC you are using to make Icy Touch fire at the same time as Death Grip to shave about 1-2 seconds per monster. (I don't use GB/HB but the cc's look simple enough)
Unholy Presence (15% attack speed is around 5% Total DPS boost with duel-wield) [Stack all the +hit you can]
Rotation: Pull with Unbreakable Armor + Icy Touch + Death Grip; Plague Strike; Blood Strike if RP<52 && no death rune; Blood Strike if RP<52 && no death rune; Obliterate if RP<52;. Howling Blast on Rime proc; Death Strike to heal; Frost Strike if RP>=52; Rune Strike;
You should be leveling on mobs 1-2 levels under you for maximum speed.
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58 Unholy:
http://www.wowhead.com/talent#jbZZfzhfxcbg0u0dhcR:Vb
Can use this right out of the box, you should be grinding mobs 1-2 levels under you. Anything 2+ higher can only be handled reliably yet slowly by blood. I don't like the way most bots handle Desolation, but if you can program the CC to keep it up then by all means get it.
Unholy Presence/Blood (30% of your DPS at this stage will be melee damage, so haste is nice)
Rotation: Pull with Icy Touch + Death Grip (pull distance 20 yards); plague strike; blood strike if RP<60 && no dr, blood strike if RP<60 && no dr; scourge strike; Death Strike to heal; Ghoul Frenzy if in combat; Death Coil if RP>=60; Rune Strike;
