Demonology enhances survivability without truly sacrificing much DPS comparative to other builds while leveling.  The differences however for those fully fel-guard specced will become more apparent once you hit 70/start PvP or PvE raiding.

Demonology is a strange breed indeed. You've chosen not to focus on either DoT spells or direct damage spells. You figure that you've got a demon and you may as well make it kick some ass. Otherwise, what good is it doing? None!

wow power leveling

Demonology gives you massive increases to all survivability, solo capabilities, and past level 50 an immensely capable pseudo-warrior pet to boot.

The imp gives you a nice additional DoT, and improving it can be potentially useful at higher levels, just not while within the leveling process, not demonology. Regardless, the initial talent of choice will be demonic embrace and likely fel- intellect, the former giving you a massive health increase and dramatically increasing the viability of life tap due to your new life pool.

What is the crown jewel to some, rather the crown jewels, will be demonic sacrifice and soul link – both will provide immense improvements to your lasting power as well as damage dealt for hybrid builds such as destruction/demonology or affliction/demonology.

Demonology talents increase your pet's effectiveness by as much as twofold and some of them enhance your pet's abilities. Take every talent you can to get the maximum efficiency out of your pet.

wow power leveling

Stamina is a very important attribute for a Demonology Warlock. Since some of your Stamina gets transferred to your pet, you're going to need a lot if you want your pet to stand up to the abuse it's going to take. Stack up on Stamina and Intellect.

Hide: You've made your pet incredibly badass. There is no need for you to take extra abuse if you don't need to. Let your pet do some wow power leveling fighting while you stand in the bushes just providing support.