for example, tailloring has some tailor only spellthread
blacksmithing lets you add some blacksmith only sockets i believe
i think jewelry crafting has some JC specific gems
and alchemists get a buff to their potions that is specifically for them.
as a warlock approaching 80, which profession has the profession only item that will give me the most dps? and what is the item?
(of course if i can buy the item, i don't have to go into that profession--the item has to be tailoring only, alchemy only, blacksmithing only, etc.)
Supposedly they're all equal now. Jewelcrafting+Blacksmithing was winning for the longest time due to the fact that the JC-specific gems were prismatic and fulfilled your meta gem requirements, making the two extra sockets from blacksmithing be available for the best colour gem for your class. The JC-specific gems are losing their "counts as blue, red, and yellow" ability which should even things out.
Obviously this may not be precisely the case. Engineering and Tailoring don't exactly provide flat increases, you know? You'd have to look at Elitist Jerks or something like that to know for certain, and it's not likely to be a "warlocks should take this..." so much as "Cookie Cutter destruction warlocks should take ______, but affliction should take ______."
Just to be clear, the spellthread tailors get for themselves are identical to the type they provide for others, just with reduced material costs. The "perk" to tailoring is the cloak enchant, which is an attack power, mana, or spell damage proc.