Already there is a claimant who says he has reached 80 with his Death Knight:
"Odds are, I'd say, that we'll see our first level 80 sometime this afternoon (and probably in Europe, since they got a little bit of a time zone head start -- we've had reports come in from one very obnoxious Paladin, but his claims to level 80 aren't actually true. But in the meantime, here's the first level 70 Death Knight we've heard of: Arello of Raptors Open Doors on Shattered Hand dinged 70 as a Death Knight in Northrend's New Agamand. The Armory doesn't have his character in yet, but this seems credible to us."
With the new client install you are able to create a Death Knight on any realm where you have a level 55 or higher character existing already. You can make a Death Knight from any race as well as your existing characters opposite fraction.
For answers to more questions about what you can and cannot do regarding a Death Knight, check out wow Eurpe's faq on the DK
What can the Death Knight do?
Here's a simple explanation by www.azerothcookbook.com/death-knight-discussions-builds-and-notes
"As it stands, their ability trees are blood, frost, and unholy. They generate runic power and use it to power their big smash attacks. Blizzard sees them as tanks for caster mobs, which brings about interesting PvP thoughts, but they’re solid in the DPS department and should be a blast to solo.
Talents? Builds? Powers? Well… All their talents and abilities are still being run through the beta grinder and will be buffed, nerfed, and kicked around until Bliz is happy enough with their status. Since they’re getting a lot of attention from the Devs I assume that the “happy enough” state will actually occur."
Wotlk Secrets. is a guide based on the Wrath Of The Lich latest WoW expansion
Here's a video detailing the Death Knights Build upgrade