While you start at level 55 when creating a new Death Knight, the quests in the new special Death Knight starting area are widely unknown. For that reason one should consider reading up about them in a good guide, in order to finish them more quickly and efficiently. By doing so you might be able to finish the quests in the new game within two hours, reaching level 58 in the process - and three levels in two hours is nothing to sneeze at. Similarly, it is also helpful to use the popular guides for the further levels, as many Death Knights make the mistake of going through the dark portal right at level 58 in order to quest in Hellfire Peninsula. While this is probably possible, it's very inefficient, as the quests in the eastern and western plaguelands are rather easy and offer lots of experience points. You are then able to reach level 60 or even 61 without leaving the plaguelands, making your journey into Outland that much easier, as you are doing those quests with a higher level. Additionally you didn't have to travel a lot, as you end your starting quests in the eastern plaguelands.

While you could use other level 60 areas like Silithus or Winterspring, the plaguelands are simply more time efficient due to the very short traveltime. Additionally there are also some very quick questlines that will provide a substantial experience boost.

If you continue following the guide the same idea will be repeated in a similar scheme in relation to level 70: while you could do the first quests in Northrend as early as Level 68 it is better to remain in Outland until you reach level 70.