One thing that has been frustrating cloth, leather, and mail wearers for quite some time is the ability for players with a higher type of armor cherry picking through their armor. In some cases, this is due to poor itemization in the intended gear range while others are abusing gear scaling and the ability to sometimes double-dip into stats. However, this will likely stop in Cataclysm for several different reasons.
Bornakk revealed on the forums that gear will start having incentives to make you wear the appropriate type of armor. This means that holy paladins who are currently rolling on cloth and mail items will have incentives to stick to plate. What are these incentives? We don't know. It could be as simple as something like the current gem socket bonus system or a little more complex like a percentage increase in a particular stat. This could even be a generic spec based set bonus that gave you something if completely geared in class appropriate armor. We won't really know until more details are released.