Warlocks are a very complex class in my opinion, as does Blizzard likes to agree in many strategy guides and walkthroughs. However, it does appear as if Blizzard wishes for warlocks to be the class that are used to really do well in player versus player and player versus environment. Blizzard said that warlocks are intended for payers who wish to win in one of their strategy guides.

Warlocks are one of the best caster DPS classes in the game. They also have many uses that allow them to join groups and be sought out to fill vacancies in raids.

Player versus Environment
While PvE'ing and leveling, it’s better to do whatever is most fun for you, however affliction is probably about the best way to level because you're going to do the most damage to targets without losing nearly as much mana and health. Demonology is the only other spec acceptable for leveling, and destruction is probably about the slowest spec as far as leveling goes.

With affliction, you spec in your dots, drains, and range. It's nice, because you can take on several mobs at once; you just DoT everything, and drain to get your health and mana back. Definitely is most necessary to use the voidwalker while leveling with this spec, because you'll need a tank to take some hits while your dots kill the mobs. The voidwalker will do some minimal damage, however the damage is not relevant to the build.

In demonology, you'll still do decent damage, however you're more likely to focus on your summons. In destruction, you focus less on DoT's, and more on direct damage spells like shadowbolt. These are much less mana efficient, and cause you to level slower in the long run because you are using more mana, and your drains are not nearly as effective in this build.

Player versus Player
After you get your felhunter, demonology is good for player versus player, however anytime before 60, affliction is not only the best leveling spec, but also the best spec for player versus player content. Your fear is improved (main focus in PvP) and your dots are increased. Since you won't have much time to cast shadowbolts, your dots are going to be the main focus of your damage in PvP.