The path of warlocks truly diverge at this stage, depending on how you invest your talent points. All three of our talent trees - Affliction, Demonology and Destruction - are very viable in both PvE and PvP; your choice is really about the play style you prefer.
Destruction gives you the power to blast away your targets with ease, but you will need to cultivate a drinking habit, much like other mana-using classes. Demonology adds to the power your minions already have, and gives you the Felguard at level 50. If you have been following the path of Affliction, you should be well aware of your relentless staying power by now.
For maximizing leveling efficiency and solo survivability, I will recommend continuing down the Affliction tree, followed by 5 points in Demonic Embrace for a build like this at level 60. This build gives you all the goodies in Affliction, and dips into Demonology for more Stamina (Spirit is a non-issue with Drain Life, Life Tap and Dark Pact).
If you want to have the Felguard as soon as possible, get this build. This build maximizes your DPS as a demonology lock and assumes that you'll be tanking with the Voidwalker initially, and the Felguard later. Feel free to drop the three points in Improved Voidwalker into other pets, if you use those pets more. Don't let these builds limit your options - try all the trees to find one you like, if respeccing cost isn't an issue. Go nuts!
A notable talent in the demonology build: Soul Link. Many PvP warlocks spec this, especially those who participate in the Arenas. When active, your pet effectively soaks 20% of all damage for you, dramatically increasing your survivability.
Your quartet of pets - Imp, Voidwalker, Succubus and Felhunter - will pretty much meet all your minion needs. Investing 41 points in the Demonology tree, however, will give you your personal DPS warrior who comes with his own Arcanite Reaper lookalike - the Felguard. You have two more pets waiting in the shadowy wings, but the Felguard probably represents the last useful pet you will get in your warlock career.
By level 60, your felguard can taunt, charge and do a sweeping attack like real warriors. This pet is certainly a signature pet for warlocks leveling in the Outland, but its utility quickly fades in level 70 raiding and PvP.
The other two remaining pets are the Infernal at level 50, and the Doomguard at level 60. These pets add to your range of neat party tricks - they behave like normal enslaved demons. Yes, you need to enslave them every five minutes at the cost of one soulshard, with diminishing returns. Some warlocks use these pets to wreck havoc in outdoor PvP, especially in early zones where the infernal can be an awe-inspiring sight to new players.
Despite the lack of utility of these two 'endgame' pets, the quest chains to get these pets are pretty fun, and I fully recommend everyone to do them. Remember to bring some non-warlock friends on the last stage of the infernal quests!
Another rite of passage for the warlock at level 60, for the unbelievably cool Dreadsteed. The Dreadsteed quests will require a trip each to the Scholomance alchemy lab and Dire Maul West. The mount is NOT free, although recent patches have reduced the material requirements substantially.
This is the perfect time to get to know the rest of the more established Warlock population. Besides their wonderful personalities, chances are some of them will still have the three reusable items required for the final stage. You'll save 250g if you get a Warlock with those items to help you. The other mats should cost you 200-300g, depending on your server's AH prices.
Debuff, DoTs and nukes
Death Coil - a most controversial nuke, learnt at level 42, also known as 'skillcoil' or 'lolcoil' (mainly by people who had been pwned by warlocks). It's a 3-in-1 instant: damage, heal and horror. The healing component may be trivial, but a well-placed Death Coil can reliably get a Warlock out of trouble, or get his PvP opponent into serious trouble. Use this with abandon when soloing like a normal nuke, you shouldn't be needing the horror portion of this spell much. In PvE group work, you'll want to watch this as your tank might not appreciate you peeling the mob off him or her. In the worst case scenario, the horrored mob might bring more nasty friends.
Curse of Shadow - this is the level 44 cousin of CoE, a curse that boosts shadow AND arcane damage. Like CoE, this debuff can contribute more damage on the whole than your damage dealing curses, depending on group makeup. In a raid with multiple warlocks, mages and shadow priests, your raid leader may assign CoE and CoS duties to specific warlocks to increase the raid's DPS.
Soul Fire - a long-cast (yawn) nuke learnt at level 48, probably only used by Destruction warlocks. With a 4-second cast (after improvement by Bane), its utility is obviously limited if your target is already beating on you. Some warlocks use it as their opener, or on a PvP target that's been CCed.
Unstable Affliction - this 41-point talent that will bring an affliction lock's DoT count on a target to five. This DoT makes dispelling warlock DoTs a nightmare in PvP, dealing a chunk of crit-able damage to the dispeller and silencing them at the same time. UA locks are very common in the Arenas.
Shadowfury - deep down the Destruction tree is this tier 9 talent, available at level 50. Shadowfury is nice, fast-cast AoE weapon for the destruction lock. Comes with a War Stomp effect stunning all targets in an area for two seconds. Obviously useful for PvP, and adds to a destro lock's PvE damage output if used carefully.
Curse of Doom (CoD) - think of this as a 1-minute time bomb. Like CoA and CoEx, this spell can be given a nice boost by Amplify Curse. Avoid 'forgetting' to amp up by using this macro:
#showtooltip Curse of Doom
/cast Amplify Curse
/cast Curse of Doom
The first line of the macro will force the button to show the tooltip and cooldown for CoD, instead of Amplify Curse. Use this judiciously in multi-target fights to increase your overall damage output, as this spell does not break crowd control abilities. By the time the group gets to burning the CC'ed mob, CoD should pop to deal a nice chunk of damage.
This curse is preferred to CoA in long boss fights, even though back-to-back CoAs will deal more absolute damage. The multiple global cooldowns triggered by CoA, however, can make room for additional shadowbolts if you used CoD instead. You'll also have one less DoT to constantly re-apply during the fight. Note that when CoD pops, your threat can spike dramatically - so get a timer and don't piss your tank off.
If your target is killed by CoD, there is a small chance you'll get a Doomguard spawn. This is one way of getting your doomguard 'pet' without questing for Ritual of Doom. Use CoD on level 1 critters to get your bat-winged badass!
What better weapon to complete a Warlock's outfit than a reaper's scythe, aptly named Soul Harvester. At level 50, pick up An Imp's Request from your trainer and you'll embark on a quest line that will send you to Felwood and the Sunken Temple (bring friends!). Do this series concurrently with your Infernal quests. Upon completion, you will have a choice of a staff, robes or trinket. The trinket is one of its kind in the game, while the other two are probably easily replaced by better gear later.
I picked the Soul Harvester for sheer coolness!
The Outlands and your last 10 levels await! ->