A friend of mine told me that you can only "turn your character into a Death Knight", thereby unlocking the ability to make them normally, if the character you do so with is a certain class. So if none of your toons are one of those classes, you'd have to roll something and level it to whatever level to get your Death Knight.
I don't agree with something like this, and I'd love to know where my friend got her info, but there is a degree of logic to it. Does it really make sense that, for example, a mage would suddenly gain the ability to wear plate and wield two-handed swords?
Edit: There are certainly at least two classes that would be able to turn into DKs, if no others; these two classes are given.
Paladins - Arthas. Not to mention all the other bazillion examples.
Warriors - Technically speaking, a warrior is just a swordsman; there's no reason why a swordsman couldn't succumb to darkness and embrace unholy powers. Like DKs and Paladins, Warriors can wear plate, wield two-handed weapons, dps and tank. Like DKs, Warriors can dual-wield.
Although it goes against what I said above with mages, you could say that warlocks would make good candidates as well, since they simply wield another form of unholy magic (demonic, rather than necromantic).
You could also say that Shamans might make good candidates, since they can dual-wield. Also, the whole point of the DK transformation is giving up on one's previous path in life and converting into the service of the Lich King (and subsequently breaking free of his control).
Edited, Apr 28th 2008 2:44pm by PsydonX