This article will discuss the specs for leveling a warlock for maximum efficiency. While this is a DPS class and there are points of debate based mostly on play style, Demonology is probably your best bet for effective leveling just because of the Felguard. Affliction is a close second, and Destruction is just plain silly. Simply by looking at the nature of the classes at their most advanced level (80), you can see which spec will benefit you the most when leveling by yourself (which is the majority of WoW players). Affliction is passive healing through DPS, much like a shadow priest, and Demonology is the absorption of damage through your pet as you down your enemy like a hunter.
You should have your Voidwalker out and about, taunting your enemies while you burn them down. Get used to that because that is what leveling a Warlock is all about. Put your first three points into Demonic Embrace and build up your stamina to keep you alive, as well as some extra juice to lifetap from. Place your two last points to get to the next tier into Fel Synergy to protect yourself from casters, which will be the hardest class to battle as you level. Grab Demonic Brutality to increase the effectiveness of your personal tank as well as Fel Vitality to further harden your skin against mobs.
Link your soul to your Voidwalker to further protect yourself. Early points in Demonology are about survival. To survive, you have to be able to get out of sticky situations, putting points to reduce the casting time of summoning your demon is a good idea at this point in the game (Fel Domination). Improved Health Funnel is important if your tank starts taking heavy damage. Demonic Aegis is passable mostly due to the fact that Warlocks cannot learn Fel Armor yet. Max out Unholy Power to boost your Voidwalker damage as well as Master Summoner to ensure there is a pet to protect you.
Mana Feed and Master Conjurer are very important. You need to ensure your pet has mana to taunt enemies as well as the recently super-buffed Master Conjurer that will improve your DPS noticeably. The next tier is available now and it is worth investing points into Master Demonologist. This becomes more important once the Felguard is obtained. Now that we have traveled down the tree, it is more about downing your enemies as it is easier to take on multiple enemies than when your lock was level 10. Demonic Empowerment is fun, and allows you to charge in and make things interesting. A point in that talent will benefit you, the cooldown on the empowerment is short.
If you're having trouble staying alive, Demonic Resilience may help you, but I would recommend Demonic Knowledge to pump out some more damage. This leaves you with an odd talent out before the next tier is available. One point in Demonic Aegis would not be a bad idea. Once the next tier opens up, Demonic Tactics really opens up the game. 10% crit in talents is really exciting. And finally, the Felguard is yours. The Felguard is a DPS like tank when soloing, he can taunt, hit multiple targets, you can heal him, and he can enrage (Demonic Empowerment). Leveling just became easier overnight.
If you really enjoyed that 10% crit increase, the why not bump up your pet's crit chance even more? 3 more points there will not only help you down mobs, but help your Felguard keep aggro (which Blizzard decided to name some crazy German name for everyone, thanks Blizzard). Demonic Pact increases your spell power every time your pet has a critical strike. Basically, you're always going to have this buff. Worth the points, especially as a path to Metamorphosis.
Once Metamorphosis is yours and you're morphing into a Demon to further tear your enemies to shreds, it may be time to start putting points into yourself rather than your pet. As your abilities allow you to hit harder, you should be able to start taking on enemies who are higher level. To do that, you need to make sure you can hit them. Two points is all I would put into Suppression in the Affliction tree. That should suffice. Bane is next on the list. Shadowbolt should be your most casted spell, it would be beneficial to cast it faster. Nemesis will allow you to morph into that crazy demon more often, seems like a good fit for me. Three points is worth the investment.
Now that the points in Bane are spent, Improved Shadowbolt looks like a good fit. Five points is well worth the investment there. That way we can bypass the next tier of Destruction and put five points into Ruin, which should send your crit damage for each shadowbolt into overdrive. The last three points can go into Shadowburn and Destructive Reach. Shadowburn is a favorite of mine. Make sure you use Shadowburn on a near dead opponent, or you'll risk losing a Soul Shard. The range is not a necessity in leveling, but the journey is basically over point wise.
That is my guide to point distribution for a Warlock. Affliction is not a bad bet either, but Demonology is definitely the way to go for maximum production in leveling. This is not to be confused with raiding or PvP specs. Those will be covered in different guides altogether. This game is massive, different ways to play the game require different specs for each character, thus Dual Specs were born.