Death Knight Gear and Mechanics.

Q: What is this rune system I keep hearing of? It is strange and confusing to me!
A: The rune system is a rather confusing energy system by comparison to the already existing ones, as it stands you use your runes for initial strikes and spells, such as plague strike, blood strike, icy touch and obliterate, to name the staple tools of a rotation, some skills cost 2 runes, these being Scourge Strike, Obliterate, Death Strike and Howling Blast.
All abilities that require one rune generate 10 runic power on use, two rune abilities give 15 (some abilities will generate an extra 5 with talents.) Runic power is then used for secondary abilities.

Q: What is a Death Rune?
A: A Death Rune is a blood tap triggered or ability triggered rune (with Death Rune mastery, Blood of the North and Reaping) that acts as any rune you need for an ability, these runes are put on a lower use priority than other runes.
Credit to Wormsborough

Q: Do Death Knights benefit from spell power?
A: Death Knights receive no benefit from the spell power stat.

Q: Should I use a Two Hander or Dual Wield?
A: Whichever you prefer, there is not one option intended to by far surpass the other, some would wish to argue that point, and in regards everybody has their points, but the design is intended that both are viable and competitive for both tanking and dps.

Q: What are these presences I hear of?
A: Presences are unique death knight self buffs, akin to warrior stances in their role assosciation yet without restrictions or negative side effects. The presences are;
Blood- A 15% increase in damage dealt and 2% self heal from all damage.
Frost- A 60% armour increase, 45% threat increase, 10% health increase and 5% incoming magical damage decrease.
Unholy- A 15% run speed increase, 15% haste and 0.5 seconds knocked off the global cooldown.

Q: What kind of gear should I aim for?
A: A Death Knight will look for the same kind of gear Warriors and Paladins do. For damage you will want strength and critical strike rating as the most important stats and also expertise and hit rating. For tanking you will do the same as warriors, defence, raw armour and stamina being the most important ones and prioritize parry over other stats for avoidance, but steer clear of block rating and block value unless there is no better option.
Credit to Rumilus.

Q: Hold on, Death Knights cannot use shields and they obviously don't turn into bears, how do they tank?
A: Yes Death Knights are unable to equip shields, but they have plenty of capacity to compensate that.
All Death Knights have a baseline ability named forceful deflection in which 25% of our Strength is converted into parry rating, this gives death knights an avoidance capability from Strength. Death Knights are heavily balanced around cooldowns when it comes to tanking in order to stay alive, I dare say they are the most involved tank with keeping their self alive.

Q: Do Death Knights get unique models and skin textures for gear?
A: Nope, sorry, outlands gear will make you look as much like a clown vomited on you as much as it did anybody else.

Q: Does Death Knight starting gear scale with level akin to the "Legacy" items?
A: Nope, they are stuck with those stats for as long as they exist, including the weapon.

Q: What does a Death Knight use in the ranged/relic slot?
A: A Death Knight uses a relic called a Sigil.