Just tell them loot rules at start, including things like "No ARP trinkets for ret pallies, or spirit items for shamans" etc.
Well really............it's all about the iLevel so the person can get a higher gs.
http://www.wowarmory.com/character-sheet.xml?r=Scilla&n=Wenger
As others have said, upgrades are upgrades.
My main is a 'lock: I've lost cloth gear to boomkins before. If it's actually an upgrade, I'm OK with that. (Sure, I'll complain IRL about my rolls... but it's an upgrade.)
In a PUG, if people are all helping down the boss, they should have a chance to win loot. Unless the loot rules don't explicitely state that certain things aren't going to fly (e.g., Mp5 trinkets to Spriests), people have no reason to complain. Give Bonespike to the Shaman; give DBW to the Ret. If people don't like that... tell 'em to run with a guild.
With all that said, even in PUGs I watch what I roll on. My healadin, for instance, almost never rolls on cloth/leather/mail gear unless nobody else does. (It has to be one heck of an upgrade for me to try to win against a lower-armor class.)