If you are interested in leveling fast and using mainly the unholy tree, this death knight leveling spec build is the guide for you.  No matter who you are, pro or noob, remember the basics when leveling, tons of food and drink, find a combo with your spells to allow you to move on to the next mob quickly, always repair/sell/clean out your inventory at town..

The Unholy tree is usually used for PvP play but can also be built for leveling, although probably not the fastest, you still dish out great dps so you can always jump into pvp here and there.  I would pretty much call this a PVE guide.  You pretty much want to disease your target with plague strike and icy touch.. Then finish it off with scourge strike a few times.  UseBlood Strike to convert blood runes to death runes for more scourge strikes.  Remember to keep unholy blight refreshed and use Death Coil for remaining runic power.  Your glyph of scourge strike will keep your procs up, but if not use Plague Strike+Ice touch if you lack procs.

Remember to summon your gargoyle during bloodlust, this way you summon a more powerful gargoyle.  Try finding a mirror of truth trinket as well which helps.

This guide as of now will get your Death Knight to 80 and ready for WoW Cataclysm.  When Cataclysm is released, check back here for the latest 1-85 Death Knight Cataclysm guide.  If your actual leveling times are slow, I highly suggest you check out the top WoW Cataclysm Leveling Guide.

A link to the talent calculator with the build can be found here or the summary is listed below.

Level 80 Death Knight (12/0/59)

Blood (12 Points)

Butchery – Rank 2/2
Whenever you kill an enemy that grants experience or honor, you generate up to 20 runic power. In addition, you generate 2 runic power per 5 sec while in combat.

Subversion – Rank 3/3
Increases the critical strike chance of Blood Strike, Scourge Strike, Heart Strike and Obliterate by 9%, and reduces threat generated while in Blood or Unholy Presence by 25%.

Bladed Armor – Rank 5/5
Increases your attack power by 5 for every 180 armor value you have.

Two-Handed Weapon Specialization – Rank 2/2
Increases the damage you deal with two-handed melee weapons by 4%.

Frost (0 Points)

None

Unholy (59 Points)

Vicious Strikes – Rank 2/2
Increases the critical strike chance by 6% and critical strike damage bonus by 30% of your Plague Strike and Scourge Strike.

Virulence – Rank 3/3
Increases your chance to hit with your spells by 3% and reduces the chance that your damage over time diseases can be cured by 30%.

Epidemic – Rank 2/2
Increases the duration of Blood Plague and Frost Fever by 6 sec.

Morbidity – Rank 3/3
Increases the damage and healing of Death Coil by 15% and reduces the cooldown on Death and Decay by 15 sec.

Ravenous Dead – Rank 3/3
Increases your total Strength by 3% and the contribution your Ghouls get from your Strength and Stamina by 60%

Outbreak – Rank 3/3
Increases the damage of Plague Strike by 30% and Scourge Strike by 20%.

Corpse Explosion – Rank 1/1
Cause a corpse to explode for 443 Shadow damage to all enemies within 10 yards. Will use a nearby corpse if the target is not a corpse. Does not affect mechanical or elemental corpses.

On a Pale Horse – Rank 2/2
You become as hard to stop as death itself. The duration of all Stun and Fear effects used against you is reduced by 20%, and your mounted speed is increased by 20%. This does not stack with other movement speed increasing effects.

Blood-Caked Blade – Rank 3/3
Your auto attacks have a 30% chance to cause a Blood-Caked Strike, which hits for 25% weapon damage plus 12.5% for each of your diseases on the target.

Night of the Dead – Rank 2/2
Reduces the cooldown on Raise Dead by 90 sec and the cooldown on Army of the Dead by 4 min. Also reduces the damage your pet takes from creature area of effect attacks by 90%.

Unholy Blight – Rank 1/1
Causes the victims of your Death Coil to be surrounded by a vile swarm of unholy insects, taking 10% of the damage done by the Death Coil over 10 sec.

Impurity – Rank 5/5
Your spells receive an additional 20% benefit from your attack power.

Dirge – Rank 2/2
Your Death Strike, Plague Strike and Scourge Strike generate 5 additional runic power.

Reaping – Rank 3/3
Whenever you hit with Blood Strike or Pestilence there is a 100% chance that the Blood Rune becomes a Death Rune when it activates. Death Runes count as a Blood, Frost or Unholy Rune.

Master of Ghouls – Rank 1/1
Reduces the cooldown on Raise Dead by 60 sec, and the Ghoul summoned by your Raise Dead spell is considered a pet under your control. Unlike normal Death Knight Ghouls, your pet does not have a limited duration.

Desolation – Rank 5/5
Your Blood Strikes cause you to deal 5% additional damage with all attacks for the next 20 sec.

Improved Unholy Presence – Rank 2/2
While in Blood Presence or Frost Presence, you retain 15% increased movement speed from Unholy Presence, and your runes finish their cooldowns 10% faster in Unholy Presence.

Crypt Fever – Rank 3/3
Your diseases also cause Crypt Fever, which increases disease damage taken by the target by 30%.

Bone Shield – Rank 1/1
The Death Knight is surrounded by 3 whirling bones. While at least 1 bone remains, <he/she> takes 20% less damage from all sources and deals 2% more damage with all attacks, spells and abilities. Each damaging attack that lands consumes 1 bone. Lasts 5 min.

Wandering Plague – Rank 3/3
When your diseases damage an enemy, there is a chance equal to your melee critical strike chance that they will cause 100% additional damage to the target and all enemies within 8 yards. Ignores any target under the effect of a spell that is cancelled by taking damage.

Ebon Plaguebringer – Rank 3/3
Your Crypt Fever morphs into Ebon Plague, which increases magic damage taken by 13% in addition to increasing disease damage taken. Improves your critical strike chance with weapons and spells by 3% at all times.

Scourge Strike – Rank 1/1
An unholy strike that deals 50% of weapon damage as Physical damage plus 400. In addition, for each of your diseases on your target, you deal an additional 25% of the Physical damage done as Shadow damage.

Rage of Rivendare – Rank 5/5
Your spells and abilities deal 10% more damage to targets infected with Blood Plague. Also increases your expertise by 5.

Glyphs

Glyph of Dark Death (Major Glyph)
Increases the damage done by Death Coil by 15%.

Glyph of the Ghoul (Major Glyph)
Your Ghoul receives an additional 40% of your Strength and 40% of your Stamina.

Glyph of Scourge Strike (Major Glyph)
Your Scourge Strike increases the duration of your diseases on the target by 3 sec, up to a maximum of 9 additional seconds.

Glyph of Raise Dead (Minor Glyph)
Your Raise Dead spell no longer requires a reagent.

Glyph of Pestilence (Minor Glyph)
Increases the radius of your Pestilence effect by 5 yards.

Glyph of Horn of Winter (Minor Glyph)
Increases the duration of your Horn of Winter ability by 1 min.

If you are having trouble leveling your Death KnightI highly suggest you check out Zygor’s Leveling Guide because it has a perfect Death Knight leveling path that gets you to 85 faster and easier than any other way.  No matter what level you are, you load the thing and it points with an arrow the direction to each monster, quest giver, trainer etc.  Its pretty crazy.  It also has an in game box that tells you exactly what to do, like kill 5 boars, run west 10 miles, etc.  Its pretty crazy!  You can find out more here or check out a review.