While Death Knights start at 55, by the time they get out of the Ebon Hold, which is the instanced newbie zone that delivers the epic story of how a Death Knight becomes a Death Knight, they should be level 57 with a few bubbles in. Level 57 is pretty cool, but it's far from the level 68 or so you'll need to get to Northrend, and it's not even the level 58 that most people consider the minimum for heading to Outland. So, what's a fresh Death Knight to do?
I'm taking my Beta Deathknight to Winterspring to start with. I figure that between Everlook, Starfall Village to the north, and Donova Snowden to the west, I should be able to get a good, solid mix of quests that'll allow me to hit 58 and decide if I'm ready for Outland -- Not to mention a Mechanical Yeti.
Of course, there's other options as well. Eastern Plaguelands offers the chance to use Rune of Lichbane and mess up a bunch of undead in the name of the Argent Dawn, plus you can have your 70 collect Savage Fronds, Bone Fragments, Cores of Elements, Dark Iron Scraps, and Crypt Fiend Parts for some quick experience as well.
With a good group, you could also hit up Scholomance and Stratholme for some revenge against the Scourge, maybe grabbing some Deathcharger's Reins or a Barov Peasant Caller along the way. Or you could trust in the power of the imba and try hitting Hellfire Peninsula straight up at 57.
Then again, this is Beta, so they could always just switch things up, add a few more levels gained in Ebon Hold, maybe. Still, it's likely we'll still have a while to go before we can hit all the new stuff, regardless. So if you're rolling a Death Knight, how are you planning to start the level grind?