I was looking through my personal bank for Leiandra the other day, and I did a pretty good job at cleaning stuff out, but I still have things that have been sitting in there for a few years now. Being that I've had most of these items since before the last expansion went live, I have a feeling that as we get closer to the next expansion release, they'll be even further diminished in their value.
As an prime example of one of the items that lost value, let's look at Mooncloth. It used to have a cooldown of about 4 days (3 days, 20 hours). (As of patch 2.4.3, the cooldown was removed, but that was long after the expansion was released.) With a cooldown of 4 days, there will generally be an inherent supply shortage. While there were a number of high-end, level 60 items that could be made with Mooncloth, Outland drops were far superior. The only reason left to create Mooncloth after the expansion hit was for the 16-slot Mooncloth bags. But even those are "replaced" with Netherweave Bags (although the Mooncloth ones have the advantage of not being BoE).
In Classic Wow (pre-The Burning Crusade expansion), the Essences (Fire, Air, Earth, Undeath, and Water) were a pretty big crafting component. They'd go for a decent amount on the AH, and many areas were farmed for it. I remember spending long periods of time farming the 10 Fire, Air, Earth and Water that I needed for my Robe of Archmage. Again... only to be replaced shortly after entering the Outlands.
So, what can we learn from last expansion to apply to the upcoming one? Start selling now. I'm not saying you need to clear out your 6 tabs in your 1-alt guild vault, but you might want to start thinking about it, and maybe even slowly moving stuff out. I'm guessing that Primals will go the way of Essences, and whatever we're farming in WOTLK may even eventually drop motes as Essences are dropped in farming motes now. So, there's really no need to keep them locked up in your vault. Start to sell the ones you won't need. And do it slowly so you don't flood the market and drop the price.
I'll admit that we don't know a lot about crafting in Wrath of the Lich King, but we can assume that there will be better stuff. You don't use level 30 items to craft level 60 gear. Neither will you use level 70 items (primals) to craft level 80 gear. You might also want to slowly start moving out leather, cloth (Netherweave, Shadoweave, Primal Mooncloth, and Spellcloth), and ore. Don't get rid of all of it; you'll most likely need some for your first skill-ups above 375, but if you have an entire guild vault tab dedicated to Spellcloth for your one level 70, you might want to think about moving that stuff out. Also, the "expected" release date is still 4 or 5 months away. You may still be using some of that stuff for yourself before we all storm Northrend. You know your situation, act accordingly, but don't let the value of those items drop too much as we get closer to Wrath.
Side note: I recognize that a lot of people hold on to lower crafting materials for alts and whatnot. That's fine. Continue to do that if that's your thing. But even as Essences lost value (both monetary and skill up value), so will Primals. Will anyone think of crafting a 400g 20-slot bag when they may be able to do the same bag for considerably less materials from Frostweave?