You know what really grinds my gears?
The fact that after seven years of WoW, people can still leave dungeon groups right after they get their item, or ninja items that other people deserve more.
Solution #1 - Quitting Groups
If a group doesn't consist only of you and people from your friend list or guild, then the game will check for the first person to click 'Leave group' within 5 minutes of getting a superior or epic item and before the dungeon has finished. That person will lose the item he just looted as he quits the group.
In addition to this, if a a player finishes an entire dungeon (since his or her time of joining the group), then that player receive a 10% chance of receiving a final bonus item appropriate to their class and specc, randomly selected from all current level dungeons. After a second dungeon, this will increase to 20%, and so on, until you're at 100% at which point you'll receive an epic item. This buff should have a 24 hour cooldown, so then you'll have to start working on it again, and perhaps it should also reset once you receive an epic item.
Solution #2 - Ninjas
When Blizzard level designers create a new dungeon, each item they implement as loot should be categorised as appropriate for agility users, tanks, dmg casters, and so on. As soon as an item drops from a boss, only people who can genuinely benefit from it will be allowed to roll. Once they've all selected greed or turned down the roll, anyone else may roll as well.