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.
This also comes on the heels of the post Bornakk made at the start of the thread warning that a lot of the tricks people use to eek those few extra dps through itemization will stop working in Cataclysm due to the stat overhaul. While this is technically old news, it doesn't hurt to remind people. Attack power as a stat on armor is completely going away (with the possible exception of things like rings, necklaces, and trinkets). So, any plate damage dealers currently strutting around Dalaran in leather chaps might want to start building a second set of gear for when the expansion hits or they'll be sorely disappointed when trying to level up to 85 using inappropriate gear.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.