You might think that you could always tell a demonology warlock on sight, its minions being bigger and more powerful, to reflect his greater dominion of demons. This isn't really the case though, every warlock's minions look the same irrespective of build. If they are over level 50 then they become easier to spot as you'll find that they'll have a felguard following them about. To be fair, he is bigger and more powerful than all the other minions.
If you're drawn to demonology so that you can summon a greater variety of demons, or maybe more exotic ones like those beastmastery hunters, then you're going to be somewhat disappointed. You'll have and use the same demons until you learn to summon the felguard then you'll have him. That said, the demonologist has an array of talents that really amplify your minion's power and also boosts your own by drawing on the power of your demon. If it is synergy with an embodiment of evil ripped from the nether that you are looking for, then this is the place for you.
All warlocks are essentially a full group all on their own, but this is never more true than for the demonologist. Your summoned demon is better able to tank and you are better able to keep them alive all while you beat the hell out of whatever gets in your way. While the demo lock has to kill things fast and the afflock kills things slowly you just wander through whatever comes along and rip it apart - be it a pack of annoyances or a massive elite that is supposed to need three people to kill.
While the demon becomes your strongest asset it is also your weakest link. Without them you are very exposed having not only lost their power and abilities (such as getting hit for you) but your own abilities are diminished by their absence. Management of the minion to keep it alive is a large part of playing a demonology lock. As well as this, any raid boss that gives a buff to the raid typically forgets to buff the minions, so you lose out on your total DPS gain there.
Stats to look for
As ever, hit is vital when taking on any creatures of a higher level then yourself (such as bosses) -- this makes it the biggest DPS gaining stat until it is capped. Unlike affliction there are no plus-hit talents in the demonology tree, so you'll be looking to gear all the way.
After hit it's all about the spell power. This has three main benefits; it gives you a big damage bonus to your spells, it boosts the damage output of your minion and it improves the potency of then Demoic Pact buff (when you get that). Beyond that, spirit is good as it also boosts spell power and haste is about as good as SP at boosting your personal DPS.
Stats to avoid
As with all warlock builds, you don't need melee or regeneration stats. Your summoned demon will not gain from any increase in your attack power, even if they are a melee demon, they feed off your spell power.
Leveling as demonology
Demonology is a solid build to level with though I find that your minions are typically fine with whatever you throw at them in whichever spec you go with. Once you get to level 50 and open up that felguard then things become much easier for the demo lock. He really is a leveling monster.
Generally you will spend most of your time sending in your tank (voidwalker or felguard) and blasting away. This is very similar to affliction but as demonology you should be able to do more damage without pulling aggro and your minion will be able to take more of a pounding.
Your basic demonology rotation
To the right you can see a basic flow of spell casting priority for raiding with demonology. There's not a great deal about managing your minion in there so if you need to do more than simply tell him to attack the target then you will have to fit that in. You wall see that after ensuring your Glyph of Life Tap buff is active the next thing you should do is ensure that Demonic Empowerment is up. This is instant cast and does not trigger the GCD, so I have it as part of a macro on all my main-nuke spells; like this:
Obviously you can also add in any other bits there to trigger trinkets etc.#showtooltip Shadow Bolt
/cast Demonic Empowerment
/cast Shadow Bolt
Whenever Metamorphosis is available pop it and trigger Immolation Aura. Now, that is assuming the fight is consistent throughout, it's a whole art form to making sure you pop Meta at the optimal time but try and get as many uses of it as you can during the fight.
Curse of Doom is your main curse but if the target isn't going to last long enough to have it trigger then you can swap to Curse of Agony. CoD is only a slight DPS increase over CoA but it's a lot easier to maintain.
Finally, you have three main nuke abilities, Soul Fire for when Decimation is active, Incinerate for when Molten Core is active (but Decimation isn't) and Shadow Bolt for all other times. It can be hard to keep track of when these buffs are up. I use TellMeWhen but Power Auras Classic can also be used.
Over the page for a look at talents and enhancements.