Warlocks are a fun class to play, especially if you know how to play them. However they're not the easiest class to work around.

What stats are most important for you?
- Spell hit: Hit cap is 202 for warlocks. In order to be effective DPS you will need to hit this.
For example a lovely piece of gear drops, 30 more damage! but you would lose 10 hit... it's not worth it.
1% Spell hit = 12.6 Spell hit rating.

- Spell Damage: Very important to any lock, but most especially affliciton due to DoTs.

- Spell Crit: Not important for Affliction locks and you shouldn't EVER gem for it unless you're meeting a meta requirement or something.
1% spell crit = 22.1 spell crit rating

- Spell haste: fairly meh for warlocks because the way it works off of damage and crit. So unless you have a fuck ton of spell damage and crit, I wouldn't worrry with it.
Having gear with spell haste, like a lot of good caster pieces, won't hurt you but the other stats are more important. On the other hand Haste scales very well for Destruction warlocks which is why you are encouraged to go destruction once you get your 4set T6 bonus.

- Spell penetration: I don't want to see it on your gear in raids. That belongs in battle grounds and arenas.

- Stam and INT: these will come directly on your gear so there is no need to enchant or gem extra for them.

Enchanting: Enchant for spell damage as much as possible.
For your boots go for Boar's Speed.
Subtlety on your cloak
On your weapon you should put Major Spellpower or Soulfrost

Meta Gem: Chaotic Skyfire Diamond. Period. Be sure you have the gems in the rest of your gear to meet the requirements.

Ok now let's talk about specing:

Once you hit the T5 level you should go for a 40/0/21 Affliction/Ruin or 41/0/20 for the Unstable Affliction build.

Rotation:
Affliction:
The warlock rotation is a little difficult to explain because they are all on different timers, so the order in which
you put them up is not the same order in which you will refresh them. Your DoTs should be up all the time on bosses.
They need to be ticking constantly b/c it is the major part of your DPS as an affliction warlock. So you want to refresh a DoT
has soon as the one before it drops off. Between them you may cast shadowbolt, or lifetap (as long as there is not a lot of raid damage going on at the time)
or dark pact as you need to.

What order should your DoTs go up?
You will generally have an assigned curse depending on the raid group. Most often it will be curse of elements for caster DPS or Curse of recklessness for melee.
This should be followed by Immolate, Siphon Life, Unstable Affliction and Corruption. If you cast UA and Corruption together they will fall off together making them
easiest to refresh. =)

Watch your DoTs and make sure that they don't fall off. Immolate has a cast time, so you should consider the cast time and begin casting it BEFORE the DoT actually drops off.

If you don't have time to get off a shadowbolt before a DoT will need to be refreshed, Life Tap or Dark Pact... but NEVER interrupt a shadowbolt to refresh a DoT because it will do more harm than good
and will screw up your rotation.

Reasons you may consider dropping a DoT:
- Bosses have a 40 debuff limit. So if a buff is getting pushed off Drop your Siphon Life.
- If there is no fire mage in the raid you may drop your immolate.

Please not that for maximum DPS it is always best for you to keep up all of your DoTs all of the time!

Felgaurd Spec 0/41/20 or 7/43/11
I don't recommend this spec if you are new to the warlock clsas and are not already familiar with it. It is difficult to maintain
and required a lot of work with your pet. It also damages your DPS on bosses that you cannot use a pet.

DS/SnF 0/21/40

This is the end game raiding spec but many switch to it before that know how to use it or have the gear for it.
To use this spec you MUST MUST MUST have hit cap. If not you're a waste of a raid spot. The warlock hit cap is
between 196 and 202 +hit. You should have at least 1100 shadow damage and around 25% spell crit after talents if you want to use this spec in raiding.

Added (2009-07-16, 1:48 Pm)
---------------------------------------------
Ok now you have the Shadow/Fire, Straight Shadow and Straight Fire Specs.

The Straight Fire spec is fairly useless. Don't Do it.
Standard Destruction Spec... Has full fire talents and is useful in early T6 content. Immolate should always be kept up
and shadow bolt is your nuke.
If you want more information about specing for destruction visit: http://www.gamefaqs.com/computer/doswin/file/928901/51867

Straight Shadow is decent once you have a 4pc T6 bonus and have begun collecting sunwell gear.

Helpful for sunwell is the talent of Nether Protection due to the fire damage in sunwell.
Soul Leech will help out the healers a bit because of the large amounts of AoE damage hitting the raid.

ONE MORE THING. Threat.
1 point of Damage = 1 Point of threat. Your crits don't cause *extra* threat but an 8k crit causes 8k threat.

HOWEVER... spells such as Searing Pain cause 2 points of threat every 1 damage point... so unless you plan on tanking be careful with it.

To avoid this you should have the subtlety enchant on your cloak and take Destructive Reach to reduce the threat caused by your spells.

If you are in melee range of the boss, it IS more likely to attack you. Once you hit 110% of the tanks threat you are screwed if you are next to him.
You have up to 130% if you are not in melee range, this also gives the tank a little time to taunt him back if the boss is tauntable.. otherwise you're
still probably going to die.

-Do our "health/mana modifying" spells generate extra threat from healing?
Dark Pact - Yes (.5 threat per mana gained)
Death Coil - No
Demon Armor - No
Drain Life - No
Drain Mana - No
Fel Stamina (from DS Voidwalker) - Yes (.5 threat per health gained)
Fel Energy (from DS Fel Hunter) - Yes (.5 threat per mana gained)
Health Funnel - Yes (.5 threat per health transferred)
Life Tap - No
Siphon Life - No

There is also Cast threat.
None: 0 threat upon cast
Corruption
Curse of Agony
Unending Breath

Low: around 75 threat upon cast
Demon Armor
Detect Invisibility
Shadow Ward

High: around 115 threat upon cast
Curse of Doom
Curse of the Elements
Curse of Recklessness
Curse of Tongues
Curse of Weakness
Siphon Life

For other information on playing your warlock please read: http://www.wowwiki.com/Warlock_PvE_guide
There is also information here for those you that are playing with warlocks in your raid. =) It makes for
happy coexistence if you know what your fellow raiders are thinking!