Yipeeeee!!! Level 68 gives you 'permission' to head to the fertile xp grounds of Northrend. Given the increased experience from questing and killing mobs, you'll hit 70 before you can say "Death Knights are OP!"
You won't get a lot of new abilities, but as you add talent points to your blood tree, you'll find it a breeze to solo your way to max level, or find a group of buddies and farm instances for the same.
So, congratulations! You've made it to Northrend!
Whether this is your first time here, or your 8th alt, take a few minutes to enjoy the scenery. Northrend has some truly incredible vistas to offer, from the soaring bluffs of Howling Fjord to the savage green wildlife in Sholezar Basin, to the rugged blue peaks of Icecrown. Enjoy!
Now, about getting that DK to 80......
1. Talent spec 68-80. Here we go.....
By level 77, your spec will finally get dancing rune weapon and be able to provide as much dps as your pre-raiding heart desires. :) The final points can go into blood gorged until you respec for heroics/raiding at level 80. You'll still get some cool buffs at the trainer, but nothing really new until the super-cool Army of the Dead at 80. This spec doesn't give you tons of health regeneration, but as I said previously, it should be plenty for just about everything you might encounter from the elite Jormungar in Dragonblight to packs of rhinos and tigers in Sholezar.
However, if you have been making gold hand over fist or at the very least, not spending it on tuxedos and vanity mounts, then you should have plenty of gold by level 70 or so for dual spec. I strongly recommend you select a full blood tank spec (look for non-broken link this evening), for two reasons. First, you will now be able to tank properly, which will allow you to instance farm, or at the very least easily find a group to complete instance quests. Second, you can solo ANYTHING. I do mean anything. Just as an example, by level 75 you can solo the 5-man wanted quest mob in Dragonblight that my poor level 80 heroic blues-equipped rogue still has great trouble with. If you've been gathering a defense gear set, you should have a decent crit-reduction (defense) and stamina set - even if your avoidance is poor. Any time you encounter an elite quest, just hit your tank set button, frost presence, 'activate this spec' button, and WIN. You can pick a different tank spec (frost or unholy) for instancing, but blood is far superior for soloing elites because of the massive health regeneration.
2. DPS considerations. Since you hit 67 back in Outlands, your rotation has been pretty much set.
As a reminder:
For single or two-mob pulls, you should:
Icy Touch > Plague Strike > Heart Strike > Heart Strike > Death Strike > Death Coil
HS > HS > HS > HS > DS > DC > DC
For most non-elite or instance mobs, they'll die on or shortly after the first rotation. If you're fighting a boss mob in a 5-man, then at the end of your first rotation, immediately after your Death Strike, hit hysteria, summon ghoul, dancing rune weapon, in that order. The burst from these blood abilities is wonderful! By 70, if you've selected the proper stats on your gear, gemmed any slots, and followed this rotation, you should be well over 1000 dps in Utgarde Keep - your first Northrend instance.
Aoe farming is still silly easy. By the time you are in Sholezar Basin, you can just run around and grab 4-6 mobs, aoe them down and finish at 100% health, and easily do 1300dps soloing. Obviously if you have heirloom gear your experience and dps will be even better, although I've recommended elsewhere to NOT pick up the heirloom weapons.
3. Miscellaneous. I recommend starting in Howling Fjord. You certainly don't have to - Borean Tundra is fine - but Utgarde keep is more friendly for sub-70s than the Nexus. In addition, your axe from Ring of Blood is still pretty good until you do the Last Rites quest chain at about 72, which gives you the Axe of Frozen Death.
When you first arrive, you may want to pick up the flight paths asap. If you go to Borean Tundra and start with the Valiance Keep flight path, it's pretty easy to head east along the coast to Unu'pe. From there, take the sea turtle to Moa'ki Harbor, grab the fp and continue on to Kamagua. From there you can run cross country to Valgarde and you're set with all the beginning flight paths. I also take the opportunity to get Dalaran when I can by offering "10g port Ironforge to Dalaran" which will be handy later. Don't set your hearth in Dalaran, however, which is what a lot of people do. The reason for this is you should still set your hearth wherever you're questing or you'll spend lots of time (and gold) flying from Dalaran to your current quest hub.
Do all or almost all the quests in one zone before moving to the next. Northrend quest hubs are generally pretty well-organized and let you accept and complete 3-5 quests at the same time.
I go Howling Fjord > Borean Tundra > Dragonblight > Sholezar Basin > Storm Peaks > Icecrown.
If you have a flying mount and you can purchase the cold weather flying on a main, you'll level extremely fast. If you don't have an 80 on your DK's server, you will have to ride a land mount until 77. I pick this order because the quest hubs are better organized and quest-rich. If you run a few instances, you can completely skip Grizzly Hills and Zul'Drak. If you do go to Zul'Drak, you may want to pick up the nice sword in Drak'Theron Keep, otherwise, your only foray into the zone should be to complete the Ampitheatre of Anguish to get your De-Raged War Axe, which awards insane amounts of xp as well as the weapon you will use until 80.
Well, good luck in your journey to 80!
Even if you're a min-maxer goal-oriented raider like me, don't forget to stop and smell the Adder's Tongue!