Ah, I'm known now! Sorry for the very long delay in response...I just returned from a vacation week 1500 miles away. While I could have had the internet, I chose to stay away =)
Leveling a Warlock, in my opinion, is the worst thing ever. If my Warlock was deleted and I had to start over, I'd cry. A lot. Every other class has something that is very helpful for leveling, but I fear (pun intended) Warlocks require a bit of...finesse to level efficiently. Each has its pros and cons, and I'll go over each below...
Destruction (25 or 30% increased fire damage...don't remember which)
Ability gained: Conflagrate (requires Immolate to be on the target)
Levels 10-68: Conflagrate hits hard, but it requires you casting Immolate first. If you are leveling Destruction, take the Aftermath talent that dazes enemies when hit with Conflagrate. Your rotation will primarily be Immolate, Conflagrate, and Shadowbolt until you get Incinerate. The Imp is a terrible questing partner, and as such, I prefer the Voidwalker or Succubus for these levels. In dungeons, you will only compete with other classes on bosses by applying Immolate, Corruption, using Conflagrate on CD, and spamming Shadowbolt until you get Incinerate. Soul Harvest on a 30 second CD is Blizzard's gift to you as you level your Warlock. Abuse it and its Soul Shard regenerating effect.
Levels 69-85: Chaos Bolt returns health and mana, Conflagrate hits like a truck, and enemies have enough health to allow the use of Shadowburn to return your Soul Shards and be an effective execute. Single target should be Immolate, Conflagrate, Chaos Bolt, Incinerate spam. Use Shadowfury if things get close/hairy. For dungeons, make habitual use of Rain of Fire...other classes will have so much burst and AoE that you'll be left behind. Just Rain of Fire, Life Tap, and Soul Harvest your way through dungeons.
Demonology (15% increased Fire and Shadow damage)
Ability gained: Summon Felguard
Levels 10-68: You will do one thing and one thing only...spam Shadowbolt. Shadowbolt will hit harder in this spec than Destruction due to the fire and shadow specialization, but your true power won't come until the mid 50's (I think level 54) when your Felguard gets his signature Felstorm ability. While leveling in the earlier levels, keep your Felguard out at all times and just use Shadowbolt until things die. There are no signature talents aside from the healing you get from Fel Synergy and being able to ressurect your demon instantly should he perish (and believe me, he will). Pick the talent that allows you to root targets of your Hand of Guldan ability to make questing and leveling a bit easier. Once you hit level 54 and you acquire Felstorm, use it every time you fight something. It will die, and it will die quickly. Also, when things get tricky, use your Felguards Axe Toss ability to stun something for a short period of time. For dungeons, get into melee range, use Shadowflame, use Felstorm, and then Hellfire. If your healers whine about you taking damage at the cost of much higher DPS, just use Rain of Fire. However, once you've advanced to higher levels and you get the Cremation talents that buff your Hellfire and allow you to move while casting it, politely tell your healer to shut up and you'll heal yourself out of combat with Soul Harvest. Shadowflame + Felstorm + Hellfire will prove to be a very formidable force in dungeons.
Levels 69-85: This is when Demonology becomes Demonology. You get your signature ability Metamorphosis, and from this endless opportunities arise that make leveling a joke. Metamorphosis makes you a tank...you get 600% armor, are unable to be crit by melee attacks, and you have a 50% snare and stun duration reduction. On top of that, you look awesome. Use this ability on cooldown, much like a Paladin would use his vastly inferior wings. For single target questing, use Hand of Guldan and Shadowbolt while using Felstorm religiously. In dungeons, no one should ever beat you in AoE again except those pesky Survival Hunters with their LAWLMULTISHOT instant AoE. On top of Shadowflame, Felstorm, and Hellfire, you also unlock a new ability under transformation called Immolation Aura. This is a much more powerful AoE ability than Hellfire that causes no damage to you. Make sure you use this every time you transform into a demon.
Affliction (30% increased Shadow damage)
Ability gained: Unstable Affliction
Levels 10-68: Spam Shadowbolt. Shadowbolt hits harder than anything else due to its 30% buff with shadow specialization. Enemies in Azeroth won't be alive long enough for anything else to be worth it. When you get your Felhunter, use him as an effective tank because Shadow Bite is amazing. When you get to higher levels, you can start multi-dotting, which is the phrase used for putting up multiple DoTs on more than one enemy to increase your overall output. Since you have Soul Harvest on a 30 second cooldown, you can put Corruption, Unstable Affliction, and Bane of Agony on 3 or more targets, let the DoTs kill them while you cast Drain Life on the enemy with the most health. Questing will be rather dull, and you will always be in last place in damage done in dungeons due to your lackluster AoE and slow ramp up time with DoTs. Drain Life provides a higher level of survivability, but Blizzard has made this game pretty easy to level, so you shouldn't feel pressured to level Affliction.
Levels 69-85: Cast Haunt, spam Shadowbolt or multidot when applicable. At these levels, Affliction will feel slow, especially compared to other classes. That's because it is. In dungeons, you can use a Soulburn: Seed of Corruption to do mediocre AoE if the rest of the group hasn't killed stuff yet. If you get into groups with bad players, you may be able to top meters, but don't even bother competing with Warriors, Hunters, Shadow Priests, Boomkins, Feral Druids, Rogues, Shamans, etc...you get the point. Warlocks have too many other decent options to try to level Affliction.
This would be my recommendation to level as painlessly as possible:
Start as Demonology...progress through the lower levels until you hit unstoppable status at level 54. Continue growing in power as you level Demonology. When you hit 69, enjoy your stay on top of the meters until you hit level 85. I suggest people level Demonology because it currently can be played to a very high level with a decently lower complexity than the other two specializations. At level 85, all 3 specs are very viable, but Demonology brings too much to the table to be passed up. If you have talent questions or other abilities to sprinkle in, let me know and I'll do my best to answer.