Q u o t e:
Death Knights are World of Warcraft's first Hero class. They are a hybrid class that combines damage dealing and tanking. Each talent tree for Death knights can be used for either of those roles but depending which you choose will affect their play style. Blood focuses on the physical damage dealt by melee spell, weapons and abilities with a high self-healing rate and single target DPS. Frost focuses on control elements, with a strong critical-strike/bonus-damage, as well as several talents that improve damage mitigation. Unholy heavily focus on diseases, improving summoned minions, AoE, spell damage shielding and movement improvement abilities.
http://www.wowwiki.com/Death_knight_builds
I'm not saying using Blood to tank or Frost to PvP is completely nonviable. But I'm willing to bet, when it comes to the most viable spec's:
Unholy will be more favored for it's (in a 1v1, 2v2, or 3v3 scenario):
- Movement Impairing
- Movement Buffing
- Pet Utility
- Magic counter's
Blood will be more favored for its (in a 25-man, generic boss fight)
- Sustained DPS
- Melee damage buff
- Self healing (giving tanks more focus of the healers)
- Threat reduction (albeit at the top of the tree)
Frost will be more favored for it's (in a 25-man, generic boss fight)
- Mitigation buffs
- Damage output debuff's
- AoE control (ofc only applies to bosses with adds)
- Threat increase from critical striking
And by favored, I mean the majority of talent points being thrown in that tree.
As well, in a 5v5 situation, (from what I have read about the spec's of DK's) a crit fukking frosty would be more viable in a melee train than an Unholy, as the whole purpose of a melee train is to LOLDEAD a single target down and gain the advantage of 5v4.
I'll have to ask someone from Team Laser Explosion if melee trains worked in the higher brackets.
Being Blood and being able to heal myself pretty drastically can probably win fights for me. But if something is kiting me, I'd rather have an obeying ghoul who can stun, Anti-Magic Zone, and a buffed Anti-Magic Shell saving me. While Desecration and Unholy Aura close the gap between us. And in the scenario I do die, I get to be a ghoul and exact my revenge.