I still have some investigations ongoing from the PTR: JC daily results | LW daily results | continued progression on BS at the Isle of Thunder | EG progession out there as well. In the meantime, we're closing in on the patch deadline and I thought I should put together some information while it's still actionable. I got an email from Desade with his thoughts on stockpiling for 5.2 and I'm going to add my own thoughts as well.
I'm going to rank these in order of priority:
1) Ghost Iron!! If you do nothing else, start collecting/saving/buying ghost iron. There are so many new things going in that involve ghost iron it's not funny. Blacksmiths will need it for their new daily CD as well as the new PVP gear they'll be learning. Engineers will need it for their new component which is used in all three of their new recipes, and Jewelcrafters will need to prospect it for the Serpent's Eye(3) they need to use their daily CD. That's not to mention all the ghost iron bars required for the new "improved" Blacksmith leveling process that uses ghost iron instead of all the old-world metals. I expect this to put heavy demand on ghost iron.
2) Trillium Bars/Living Steel: I expect trillium bars to get residual pressure from the ghost iron squeeze. If ghost iron becomes too expensive, we may see crafters start shopping for trillium bars to make their Living Steel. I expect a spike in Living Steel demand due to increased interest in the current epics (new Raid Finder requirements) and demand from raiders for the 5.2 epics. Living steel is also used in all three of the new Engineering recipes. And apparently trillium bars are required for a 5.2 quest involving the Black Prince (ty Christina!).
3) Exotic Leather & Prismatic Scales: Leatherworkers will need a stack per day for their daily discovery cooldown, in addition to what they might use for crafting gear. The discovery recipes will unlock crafted 458 PVP gear and epic quality items as well. Currently the item level of the new crafted epics is not known. Both of these types of leather will work for the cooldown and both produce the magnificent hide needed to craft items.
4) Windwool Cloth/Imperial Silk: I expect a modest bump in demand for these items as raiders start purchasing 5.2 crafted gear and non-raiders start purchasing 476 gear to get into the higher Raid Finder groups. If your Tailor won't be participating in the new 5.2 content, you may not need to stockpile much if at all.
5) Spirits of Harmony: All crafters should be stockpiling SoH for making raid gear in 5.2. Even if you don't plan to do the Isle of Thunder content, you can still make 476 gear to sell players who need to beef up their item level for the 5.2 raid finder.
Be prepared that once 5.2 drops all of these items will go up in price to some degree. This weekend should be a good time to start purchasing cheap materials so you don't have to pay sky-high prices later. Good Luck and stay tuned!