Well, I'm not in Beta (and no one's seen the game released yet) so I'm not the final word - but in making your Death Knight part of the Alliance or Horde (depending on race) will involve going to a major city (I think they're Stormwind and Orgrimmar respectively for each faction) to introduce yourself to the leaders, pledge your allegiance and forsake the Lich King's service. Given that most Alliance/Horde citizens will hate you at that point, I think it makes sense that reputations will probably start low for the Death Knight and you'll need to build them up yourself.
But that's just speculation based on Beta information about quests and storylines for the expansion.