Personal Experience :
a)Blacksmithing ( For sure, pre-raid,raid,pvp gear available , kinda expensive at high levels but still valuable)
b)Mining (Only if you don't have money and you needa farm for blacksmithing)
c)Enchanting (You dont really need it, Runeforging is enough for weapon enchant, you'll get your other enchants from another enchanter)
d)Alchemy (Helps farming with a steady stat boost , high cost though, potions sell like hell in AH :P)
e)Herbalism (hmm....naaaah , wont get this one)
f)Leatherworking ( No comments )
g)Tailoring ( No comments )
h)Engineering (Nice pvp addons, and nice plate helm , but i'd get it for a hunter cause of the guns, DKs dont really benefit from it)
i)Jewelcrafting (why get it, the stuff created ain't that good and you can buy gems from the AH)
j)Inscription ( Nice proffession if you want some pvp / dps improvement , i'd suggest it )
k)Skinning ( No comments,only for leatherworking)
So to sum up :
If you have money Blacksmithing and Inscription
If you don't have money Blacksmithing and Mining
From secondary i'd get Fishing and First Aid