If a hunter would rather wear leather than mail, something is wrong. That goes doubly so for a warrior or paladin. Fixing itemization would help, but it's clearly not as easy as it seems or it might have happened already. I propose a new method which can be easily applied to all gear (once the basic method is figured out), though it may be tricky to implement the mechanics.
The general concept is to make each level of class benefit less from gear, but then in the reverse order, make the gear stronger. For example, a warrior would get 10% less benefit from gear, but plate would have 10% greater stats. The exact numbers are not the important part and would have to be adjusted to give an exact balance, it this case 10% up and 10% down results in a slight damage loss. These changes would scale downward as well, so hunters and shamans work better with mail than leather, warriors and paladins work better with plate than mail and much better than with leather. For casters, mages, priests, and warlocks would have no changes. Druids would be offset by one tier, shamans by two, and paladins by three.
My first idea was to have more stats on plate and reduced stat benefits, but that would be incredibly complex, both due to changing all gear and adjusting stat benefits.
A later idea was to have a hidden anti-BoK which would affect secondary stats (AP, crit, health...) and then have the reverse on plate. But again this seemed too complex and very hard to implement.
My final idea is to have an overall damage or healing output reduction as you go up the armor tiers but then each piece of armor of the proper sort gives an increase in damage. For example, a warrior would have 8%x3 base reduction. Putting on all leather would reduce this to 8%x2. All mail would yield 8%x1. Finally all plate would lead to no changes. If you didn't pick up on it, each piece of armor/tier is worth 1%, so a warrior in one mail piece would be down 1% damage, leather is 2%, a leather and a mail is 3%. The result is that downranking armor isn't totally crippling, so dramatic upgrades are still good (I'd hate to excessively penalize people who have awful luck with drops and end up with other armor classes), but min-maxing will be much less likely to put a warrior in leather.
I apologize that I constantly said warrior instead of paladin, but in my experience I've seen a lot more warriors grabbing gear from other classes than paladins, so that's a stronger idea in my mind. I'm looking at you, warriors that took Crown of Destruction and Ghoul Skin Tunic.
Make lower tiers of armor cause reduced damage output in order to discourage the use of lower armor classes.