It might seem a tad strange now, but that big bank of yours is going to be more or less worthless once Wrath hits the streets. Just about everything is going to plummet in value. Liquidate your assets now or be in worse of a position than our real life economy is.
There are some exceptions to that of course. You'll probably want to hang onto enchanting mats, since they can still be useful and high-value in the long run. You might also considering hanging on to some herbs for those leveling inscription. Most other raw minerals however can probably be safely sold now.
The reason you want to clear out as much space as possible is to make room for all the new toys and materials you'll be getting as you travel around Northrend. Mines, flowers, leather, and cloth are all plentiful. Between 70 and 75 I had an extra 100 stacks of cloth laying around, and I'm a guy that likes to bandage after every fight.
If you're anxious about clearing out your bank you can always create a new bank alt, even going as far as to purchase a couple bank tabs via your own guild bank if you have the disposable income. But no matter what make sure that you've got as much free space in there as possible – it's going to fill up quickly in the icy north.
Are you ready for the challenges that await you in Northrend? WoW Insider has you covered with our 8-day guide to Preparing for Wrath. Also, check out our feature on AddOns new, old and outdated written especially with the expansion in mind.