Wrath of the Lich King brought us Northrend, Naxxramas, and of course, ten more levels of DoTing, Rain of Firing, Shadow Bolting Warlockery. So saddle up your Dreadsteed, and float or fly your way to the frozen north--crown of the world. It's time to get diabolic. And this time, it's personal.
Transitioning from Outland to Northrend is a lot like transitioning from vanilla-WoW content to Outland was. The mobs hit a little harder, and the gear is a little better. The step up isn't quite as sudden or as large as it was last time, but you definitely want to take a good look at some of those quest greens you're offered, and it's best not to take a level 68 Vrykul too lightly. They're kinda mean.
These last 10 levels are rather straightforward in comparison with their predecessors. If you've made it this far, then what you're going to see over the next 10 levels is mostly just an inflation of your numbers that will make you squeal and clap your hands in glee. Other than that, though, you should continue playing as you did in the later part of the 61-70 bracket. If you're leveling Affliction, DoT-DoT-Fear still gets the job done with gusto. If you're leveling Demonology, the felguard still tanks, and your nukes still burn. If you're leveling Destruction, you're still insane and should get your head checked.
With two potential places to start in Northrend, we have more choices than we were given in Outland. And as with any choice Blizzard gives us, the inevitable question becomes: which choice is better? There isn't much of an answer. Howling Fjord is certainly prettier, but as Warlocks, we have long forsaken the pursuit of beauty, so that's hardly a sound criterion. More relevant is that there are fewer undead enemies to worry about in Howling Fjord, so liberal use of Fear is possible. Then again, this is Northrend. How long do you really think you can avoid fighting a zombie or two?
If, like many Warlocks, one of your two professions is tailoring, you may seriously want to consider getting the questing achievements for both Howling Fjord and Borean Tundra back-to-back. Both are required for Loremaster of Northrend, which in turn is required for learning the pattern for the Deathchill Cloak--which is a pretty snazzy pattern. And if you don't do it now, then decide you want the pattern later on, you'll have to do what I did and spend a few days doing gray quests for 3 gold a piece. Let me tell you, I can think of a few better ways to spend a day.
From there, I would recommend moving to Dragonblight for the story events that happen there. If nothing else, you should get it out of the way in case you ever decide to attempt regicide--since failing to complete the events in Dragonblight means certain areas of capital cities will be phased differently for you than they will be for most of your group.
Grizzly Hills, Zul Drak, and Sholazar Basin are all excellent places to go after Dragonblight. But once you get to about level 77 or 78, you should seriously consider heading straight into either Storm Peaks or Icecrown. Storm Peaks is home to a reputation faction, the Sons of Hodir, who you're going to want to be exalted with once you hit level 80 so you can start purchasing their shoulder enchants (unless you're a scribe, of course.) And questing in Icecrown is required to unlock the Ebon Blade quartermaster. Since the Ebon Blade are the guys who sell the pattern for the best soul shard bag in the game right now, tailoring Warlocks shouldn't put this off for too long. Besides, the quest leading up to unlocking Ebon Hold is awesome.
Talents are the most uninteresting thing about this range of leveling. By level 70, a player has long had enough points to get the 51-point talent of their choice. After that, point assignments are really just supplemental. Which isn't to say supplemental talents aren't useful, but putting points in Fel Synergy isn't anywhere near as exciting as putting a point in Haunt is. I always find that last talent point you get at max level to be so anti-climactic. It never goes into anything cool.
At level 75, a more malevolent version of the 41-point fire Mage talent becomes available to Warlocks of any spec. In brief, Shadowflame is an instant cast AoE attack which deals shadow damage to anything standing in front of you, as well as applying a DoT which ticks for fire damage. For me, Shadowflame satiated a desire I've long had for instant burst damage that doesn't come at the cost of 11 points in the Destruction tree, or require a 3 minute cooldown. And despite the limited range and somewhat awkward control of Shadowflame, it certainly fills that void in my black, black heart.
As an Affliction Warlock who relies on the self healing effects of Siphon Life rather than on the damage preventing effects of Fear, Shadowflame is a great way to expedite the deaths of my enemies after I gather up four or five of them. Often that little extra burst of damage means the difference between gaining experience, and making a corpse run.
There is a quirk, of sorts, which confuses a lot of Warlocks. The animation for Shadowflame lasts several seconds, and during that animation you can run around, pirouette, do a few back flips, and all the while Shadowflame's animation will tag along, spewing purple fire in front of your gleeful Warlock. However, the damaging effects of Shadowflame don't actually move with you. Rather, they only effect anything which was in the area of the spell's effect at the moment you cast it. Keep that in mind if you've ever got enemies attacking you on two sides, and want to hit them both with a blast. Take a step backwards and force all your foes to stand in front of you--to await doom at your malevolent hands.
Demonic Circle (Summon / Teleport)
Once you finally reach 'the big ding,' the trainers grant you a peculiar pair of spells. They're are fun at parties, and if you're clever, actually pretty useful in the field as well. The basic mechanic is quite simple. At any time, for the cost of a pittance of mana and a .5 second cast, a Demonic Circle can be summoned where you're currently standing. Doubtless you've seen a few of these circles around capital cities, it's how Warlocks mark their territory. Once the circle is placed, so long as you're within 30 yards in any direction, you can use the Teleport spell to instantly move your character into the Demonic Circle, with a little poof of dust and shadow.
Demonic Circle is a perfect example of why I love Warlocks so much. I mean, aside from LOOKING cool, it's gotta be one of the most entertaining gimmicks added to the game in a long time. I remember spending quite a bit of time, shortly after I hit 80, summoning a circle on the ground, then flying straight up as high as I could, dismounting, and playing "chicken" with the ground--with my repair bill as the stakes.
Aside from being good for a chuckle now and again, though, Demonic Circle actually has a multitude of practical uses. When writing my guide for Warlocks in Naxx-10, it seemed as though every boss in the raid could be made somehow easier by judicious application of Demonic Circle. And while I'm well known for being terrible at PVP, the fact that teleporting can get you out of snares, or help you run the flag in Warsong Gulch, is quite a boon.
Allow me to be the first to congratulate you! Eighty levels is a long way to go, but now you've made it. Time to start getting a feel for how your Warlock will play at max level!