I'd have to say, for WotLK + Cataclysm, the top boxing team from a PvP perspective has been 4x Death Knights (Frost) with a Healer.
If you're running Arena, the top choice is a Paladin but a Holy Priest is good because they can continue to heal after their death.

The main DK attack is Howling Blast (Frost Rune).
With talents and the glyph, you're hitting everyone within a 10 yard radius at a range of 30 yards from the DK.
Anyone hit has Frost Fever on them, which besides being a DOT also snares them to 50% movement.
Most of the DK abilities do additional damage, based on which diseases are on the target.
Necrotic Strike (Unholy Rune) does damage and negates some healing the target would receive.
You can 2x HB and 2x NS, plus 2x (or 1x + 1x) either or the other ability on a full set of runes.

You also have Obliterate (Frost + Unholy), which is single target and hits really hard.
And hits harder with more diseases on the target.
This combines well with Hungering Cold, which is an point-blank AoE freeze, like a Hunter's Ice Trap.
Disease damage does not break this, but any other damage will.
You can 3x Obliterate on a full set of runes.

Death Strike is the third attack option.
This can heal the DK's for a decent amount, but once your healer is down... it's not a good situation regardless.

DK's have two resource mechanics.
They have their Runes, two each Frost, Blood and Undead.
Frost spec DK's always have their Blood Runes count as Death Runes (wild-card runes).
They also have Runic Power, which is like Rage... you gain it with the use of your Rune abilities.
Frost Strike (melee range) and Death Coil (ranged) are both Runic Power dependent moves.

DK's have a few cooldowns, which in battlegrounds can be used on demand, but in arena can be used preemptively.
- Icebound Fortitude breaks Stuns and increases their damage dealt.
- Pillar of Frost makes the DK's immune to knockbacks like Typhoon or Thunderstorm.
- Lichborne makes the DK's immune to Fear, Charm and Sleep effects for the duration.
- Anti-Magic Shell reduces spell damage taken and prevents the application of any debuff with an icon for its duration.

The DK's essentially negate snares/root effects uses against them, because of Death Grip.
They can be snared, but Howling Blast will snare for an equal amount which essentially means being snared is not a penalty.
And while rooted, they can Death Grip anything into melee range (plus Howling Blast and Death Coil are ranged attacks).

Here's a write up of my DK method.
http://www.dual-boxing.com/threads/3...n-DK-Manifesto