Blizzard has allowed every race to be a Death Knight, even if one didn't deem it possible at first. The only requirement to creating a Death Knight was having a character of at least level 55 on the same realm. In the meantime this restriction has been lifted as well, and you can create a Death Knight on any realm, as long as you possess a level 55 character on one of the realms.

The fact that each and every race is available for Death Knight creation doesn't make the decision of which race to select any easier. Of course you can leave the decision to your aesthetic preferences and your taste, and a gnome Death Knight with pink hair might have its merits, but sooner or later you might ask yourself, if a certain race has a distinct advantage.

On Horde side the Orc is undoubtedly the first choice in PvE combat with his axe specialization. For PvP fights the Tauren's additional hit points or the Undead's will of the forsaken are a distinct advantage as well.

Alliance Death Knight have different decisions to make in class selection. Humans are a good choice for both, PvP and PvE combat while Night Elfs can shadowmeld for some interesting PvP options. Draeai Death Knights have a strong group buff, which makes them a nice choice for PvE as well.