Since the Mages are seeing love, it's time to bring back some attention to the Warlock forum. I'll just be covering Affliction and Destruction first, as they are the two specs I prefer. The Demonology Meta-dance can be fun, but it's not my cup of tea.
Stats
The Stats preference is as follows:
Hit (to cap) > Spell Power > Haste > Spirit > Crit
That's the simple picture. It is more complex than that, though.
You want to be hit capped since our rotation requires the initial administration of spell effects. While some will tell you that you can compensate with higher spellpower, that is not necessarily the case. For example, Shadow Embrace (http://www.wowhead.com/?spell=32394) requires you to hit with Shadow Bolt or Haunt. Conflagrate (http://www.wowhead.com/?spell=17962) requires you to have an Immolate (http://www.wowhead.com/?spell=47811) on the target, and the damage of Incinerate (http://www.wowhead.com/?spell=47838) is increased with the Immolate effect. Since the former passive and latter active are both part of your DPS, it is essential that you hit with your spells to ensure your rotation and output are solid. For Warlocks, the hit cap is 446, or 368 with Suppression. Yes, you will see some Warlocks just spamming Incinerates and Chaos Bolts on the target and living with some misses, but you don't want to be that guy.
Spell Power is the straight up fuel to your spells. Does not need much more explanation.
Haste is often overlooked by Warlocks. This is a mistake. Haste is absolutely an important stat as casting time on our direct fire spells as well as Corruption is affected by Haste. While you will never stack Haste to the point you reach the GCD cap (1s, which is 1640 Haste), you should not stack nothing but Spell Power and ignore Haste. Yes, you will see the same guys spamming Incinerate and Chaos Bolts totally stacked with Spell Power and very low haste doing insane amounts of damage on some bosses. Put them in a mobile fight, though, and you'll see them suffer.
Spirit converts to Spell Power via Fel Armor (http://www.wowhead.com/?spell=47893) and Glyph of Life Tap.
You will often see Warlocks pushing for the highest Crit rating possible. Don't.
I neglected to update this topic. Will try to make time this week.