As expansion packs have been released and level caps increased, there have been many changes that have sped up the leveling process. This is all for the good, though it does mean that you find yourself with a bunch of gray quests sitting in your log and half a ton of items that you've forgotten who you collected them for. This is never more true than when you supplement any questing with regular trips into the dungeons of Azeroth. These places are total XP-fests, with loads of elite mobs and bosses to kill and a bonus for having used the DF. You're gonna be clearing out grays while cackling your way into the next zone.
It's not all about the XP, of course. There's gear to be had, too -- not only the drops you can get from the bosses, but each time you use the DF to complete a dungeon, you will receive a Satchel of Helpful Goods containing a blue item. These goods will often not be overly helpful (except to enchanters; shards, ahoy!), since they are randomly enchanted (meaning they have stats that are as likely to be aligned to sword-swingers as to you). On the other hand, if you get lucky (or farm them a lot), you can get some very nicely itemized items. They will also always be cloth, so even if they aren't exactly the stats you are looking for, you can at least wear them.
The sort of stats you are looking for doesn't vary too much throughout leveling. You will always get a nice boost from spellpower. Haste and crit are also decent stats for improving the damage you can deliver. Spirit was preciously more useful when Life Tap scaled on it, but now it only really helps when you have GoLT. Stamina and intellect are also prized stats for the leveling 'lock; not only do they help with your survivability out in the world, they help with your mana efficiency in groups, as well.
Inside the random dungeons
So what's it like in these dungeons? Well, when I was leveling my first 'lock through vanilla content, these places were hard (queue generic "old guy" music). They required co-ordination and awareness of the various mob and boss abilities, and they typically took a load of time to get to them. These days all that has all but gone. For a start, the DF takes away the need to travel to the dungeon; once the group is set, you get to teleport right there. A part of me feels this is a shame, as the whole thing feels less like an event -- but on the other hand, who cares, since we're here for the XP, loot and the killing of stuff, right?
You will also find that there is almost no need to know what is going on with the mobs and bosses you come across. There are exceptions, without doubt, but most are simple tank-and-spank affairs. That said, being someone that appreciates these things, I would have to suggest at least a little research into the bosses you will face and what tricks they do have. If you don't and you're going somewhere new, then it might be wise to ask your group if there's anything you need to know. Generally, they will make you aware of anything particularly nasty.
That brings me on to something important about using DF to level: how nice people are. Now, this is a random group, so I'm certainly not going to tell you that everyone will be nice. (You may even find yourself in a group with a mage.) However, I find that there is a distinct contrast between the groups I join for level 80 heroics and those in there for leveling. I guess there's less expectation on everyone. There's certainly no Gear Score elitism. There is often at least one person who is playing an alt and has done it all before. They typically seem to be happy to help, rather than berating others for not knowing everything.
As with any random group, you probably need to get a feel for them to know what you can and can't get away with. When I say this, I don't mean "Can I go on follow while I make a cup of tea?" -- I mean things like the tank's threat generation or the healer's ability to cope with a self-harming warlock. Generally, the tanks will be of the sort to grab as much as they can and let the DPS AoE the lot while the healer keeps everyone topped off. In this case, the XP will really flood in (every elite mob bringing a group modified XP gain of about the same as you killing a non-elite on your own). Other times, the tank or healer can't support this approach and you need to be a bit more careful.
If you are pulling entire rooms, then break out your Rain of Fire. It can take a little while for your tank to stop running about the place, so while he runs, you can drop a few instants (a curse or Corruption), then place that big green reticle and let the fire fall. The way threat works (read about that in the voidwalker article), you are least likely to pull aggro from the tank if you are at range. However, if you find that you often have one wander over to you at some point, then it might be wise to stand with the tank when you AoE. The benefit of this is that while you may still get hit, the tank has a better chance of grabbing the mob back (if he notices) and the mob will still be in the pile of AoE and so not last too long.
If you are having to take things a little more slowly, you can still do significant damage. You could follow the tank's target and just throw Shadow Bolts at them or, if the chance comes along, you can drop CoA and/or Corruption on each mob they grab. This can become as dangerous as Rain of Fire, so you have to play it carefully, but it sure works when done properly. As with any situation, you need to determine whether your DoTs will run their full course on a target. If they don't, then they might not be worth casting. Pretty much all bosses are worth putting all DoTs up on, however, as DoTs will typically run their course and it helps the tank keep hold of them.
While you may have rolled a 'lock for the solitary lifestyle, I strongly recommend using Dungeon Finder as a means of gathering a group of saps you can use to gain your power more quickly. The lack of need to travel, coupled with the Luck of the Draw buff and all the other benefits, make the DF a really powerful leveling tool. Just remember that if you zone in having just cleared out a cave of mobs, the chances are that they will have respawned when you zone out. Might be worth summoning that voidwalker before you leave.
Blood Pact is a weekly column detailing DoTs, demons and all the dastardly deeds done by warlocks. If you're curious about what's new with 'locks since the last patch, check out WoW.com's guide to patch 3.3 or find out what's upcoming in Cataclysm from the BlizzCon 2009: Class Discussion Panel.