Nethaera
With availability of the level 90 character boost, we feel the requirement to have a character level 55 or higher to unlock the Death Knight as a playable class is no longer relevant. The plan right now is to remove the limitation sometime next week, at which point players will be able to start a new Death Knight (which still begins at level 55) without first having to reach level 55 on another character.
Crithto
Upon looking into making this change, we discovered that a few client-side adjustments would need to go through in order for it to work. Therefore, our new plan is to remove the level 55 requirement for Death Knight creation in an upcoming content patch.