Starting the DK at 55 isn't so much a favour to the player, as a favour to the dev team. They don't have to balance them all the way through the old world, worry about when to introduce new spells and abilities, or even really have to organize the talent tree relative to when those spells and abilities are available. They can just go about building a class around the end-game, be it PVE or PVP and not worry about the grind up virtually at all.
Take a look at the talent tree for the death knight. In the very first tier it has talents for Ghouls, Diseases, all three aura 'presences', spells called Icy Touch and Mind Freeze, and Blood Strike. That would be quite a bit for a level 10 character, considering that's just the stuff that is talented and Warriors and Druids don't get all their stances and forms nearly that early.
Take a look at the talent tree for the death knight. In the very first tier it has talents for Ghouls, Diseases, all three aura 'presences', spells called Icy Touch and Mind Freeze, and Blood Strike. That would be quite a bit for a level 10 character, considering that's just the stuff that is talented and Warriors and Druids don't get all their stances and forms nearly that early.