There's some merit to be had in doing a little prequesting with your Death Knight. We've discussed this a bit in the past, but here's a quick and dirty guide to quest items you can get now. With all of these, it's a good idea to turn the quest items first, then start doing other quests. That way you maximize your rep gain. If nothing else, it'll make getting reputation accomplishments easier later.
Unidentified Plant Parts: You'll need 360 of these to hit Honored with the Cenarion Expedition. They seem to be staying pretty cheap on the AH even now, so you might want to check it out, or go farm them yourself: I prefer killing Mudfin Frenzies in Serpent Lake for them. Turning in the parts is good for two reasons: Each turn in of plant parts gives a chance at an Uncatalogued Species, which is some nice extra experience. In addition, it sets you well on the way to getting the reputation for an Arcanum of Ferocity. If nothing else, the Arcanum is a nice bonus for your first few Northrend helms.
Arakkoa Feathers: These are a little steeper, costing 1-2 gold a feather on my server, on average. Also, you'll need around 1080 feathers to cap out the quest at Honored with Lower City. Lower City's reputation vendor doesn't offer much of use, so you can always skip this one with little guilt. Of course, if you're lucky, you might get a Ravenguard's Baldric of the Battle or Ravenguard's Greaves of the Battle out of the caches, which might make it worth it.
Oshu'gun Crystal Fragments: 120 of these will let you hit Friendly with the Consortium. These are everywhere around Oshu'gun in Nagrand, so don't be afraid to go out and grab them yourself if you have some spare time this afternoon -- I'd bet it wouldn't take you much more than half an hour if you work at it. Now admittedly, there's not much of a reason to grab these or rep with the Consortium besides the fact that Ethereals are awesome, but hey, that may be reason enough.
Aldor and Scryer signets: Now, I honestly am skipping repping with the Aldor and Scryer for now, just because I like the idea of being able to move where I please in Shattrath. However, if you want to get reputation with one side or the other anyway, such as because you want a crack at the shoulder enchants for leveling purposes, it might be a good idea to lay up 240 Firewing Signets or 240 Marks of Kil'jaedan. These will allow you to hit Honored with your chosen faction straight away. Which reminds me, you'll then want to have 2 Arcane Tomes or 2 Fel Armaments for each shoulder enchantment you want.
You can also just skip it and go for Sunfury Signets or Marks of Sargeras if you're not too worried about grinding to exalted. Firewing Signets and Marks of Kil'jaedan cut off at Honored though. By the by, if I was going for a side, I'd go for the Scryers. They have a better stocked tier, and the shoulder enchants affect both melee and spell critical strikes, which is pretty sweet for Death Knights. Also, Sunfury Signets are also a lot easier to get thanks to the Blood Elves of Quel'danas. Heck, if you want to go Scryer, it's probably easier and cheaper to skip the Firewing Signets and go straight to the Sunfury.
Cloth: If you're looking to get the Ambassador title, a cross-racial mount, or just want better prices in Ironforge, you'll probably want to start in on the cloth quests. Turning them in in all the capitals should net you some pretty nice experience too. If you plan to turn in cloth for every race, that's 300 each of
Argent Dawn Reputation: Now, the Argent Dawn quests right outside Ebon Hold seem like a no brainer, except for one problem: They require you have Honored Reputation first. That means you will likely have to go do a bit of Cauldron-work in Western Plaguelands before you can turn them in. Me, I'll be breezing on by to Outland once I get King Wrynn's blessing. But if you want to hang out on Azeroth a while longer, you can gather some stuff for some quick experience once you hit honored. Namely, you'll want 30 each of Bone Fragments, Crypt Fiend Parts, Cores of Elements, Dark Iron Scraps, and Savage Fronds.
Also, If you're headed to Outland, on your way down you could always stop by the two Blood Elves and grab yourselves some nice consumables. For those, you'll probably need to kill stuff in the Blasted Lands yourself until you get 14 Vulture Gizzards, 11 Basilisk Brains, 5 Snickerfang Jowls, 6 Blasted Boar Lungs, and 6 Scorpok Pincers.