I am trying to phrase this question in a way that makes it not subjective. I am not looking for "the most fun" or "best" profession for a Warlock to practice in WoW. What I am looking for is to get more information on what profession will benefit a warlock the most in terms of content progression. Be it getting additional stats on armor or being able to create potions - what will help the Warlock the most if they invest their time in? I just recently hit level 85 and after reading about all of the professions, I am still not quite sure as to which will help me the most.
The best profession, bar none, for content progression is Blacksmithing. The two additional sockets can be fitted with epic gems that provide a +100 stat point bonus to your Intellect, more than any other profession, and Intellect is the most important stat for a Warlock. That's because it was originally balanced around non-epic gems that only provided +40 Intellect each.
All of the other professions are balanced around providing ~80 stat points. If you compare the ring enchants (Enchanting), the upgraded Jewelcrafting gems, etc etc etc, you will find that the majority offer a selection of stats but all will total around ~80. For your second profession, to maximise your progression stats, you should pick a profession that will give you +80 Intellect. These are:
The worst to pick are the gathering professions because while Skinning (+80 Critical Strike Rating) and Herbalism (+480 Haste Rating for 20 seconds every two minutes, averaging out to +80 Haste Rating) give you DPS stats, they are not as beneficial as pure Intellect from Blacksmithing or Jewelcrafting, for example, and Mining (+120 Stamina) does not provide a DPS stat at all.
Have a think about what every profession can grant your character and break it into groups like these:
You can also look at what would be the easiest to level, perhaps your race has a racial bonus to a particular profession?
Another thing to think about is, with 2 gathering professions you could just make lots of gold to buy the things you require, so funding your needs via gathering (mining, skinning, herbalism).
Having a good read through the wowwiki page will help you get a grasp too.
NB: Enchanting, Jewel crafting are big winners for a class, because they have unique crafted items only the crafter can use. E.g. JC unique gems, Enchanting rings for the crafter only.