According to the recent buffed.de interview it seems the Death Knight class could be unlocked around the 55-60 level range. Runes combination will unlock different abilities for the Death Knight. You can use up to 6 runes and there isn't any restriction (6 unholy, 2 unholy 2 frost 2 blood, 3 frost 3 blood, etc ...). For example you will need blood and unholy runes to use "Army of the Dead", and won't be able to use it if you've only got frost runes. Feel free to add on to this thread!