It's been said that Death Knights are supposed to be a specifically recognizable class, part of their heroic bearing coming in that distinctness. We've already seen some of it with the inclusion of glowing eyes, and now there's more implemented with these beta builds.
All Death Knight Races have the option of taking on a selection of darker skins and gaunt, undead, or angry looking faces. In addition, the Death Knight's glowing eyes now look more more integrated and natural on almost every race, in my opinion. The changes look better on some races. Humans, especially, look very much changed, to the point where Human females can look a lot like Forsaken females without the hunchback or protuding bones.
You can see some of the new looks in the Gallery below, courtesy of tipster Damir.
Gallery: New Death Knight skins
Death Knights also got another small cosmetic change in the form of new runes. The new runes are a bit bigger and more distinct, and I personally find them easier to keep track of. I'm glad to see the change. You can see the runes here (thanks to tipster chaud and wotlkwiki for that picture).
EDIT: There are no Forsaken pictures in the gallery because there do not seem to be any new looks for Forsaken Death Knights. This is understandable -- They're already dead, after all. You can check our older Death Knight gallery for Forsaken shots, if you wish.