In a comment this week Melmoth linked to an interesting blog article by Tomas Rofkahr about Intermittent Variable Reward (IVR), the principle that you are willing to do something unprofitable repeatedly for the small chance of getting a big reward. It's what makes people buy lottery tickets, but is also what makes people go to the same dungeon repeatedly until they get the reward from the loot table they want. I know a warlock from my guild who went to Shadow Labs over 20 times to get his Robe of Oblivion from the end boss Murmur there.
Now I don't mind a bit of randomness in loot tables, it's okay if I don't know what will drop when I kill a specific boss, or even if I come out of a dungeon and nothing for me dropped at all. But what I do find annoying is if something drops that nobody in the group could possibly use. The piece of leather armor, with no leather wearer in the group. Zoso complained about that recently (thanks again to Melmoth for pointing it out). In my only heroic instance run up to now the final boss dropped an epic, but it was mail armor, with no mail wearer in the group, and no disenchanter eiter, so I ended up winning it with a greed roll and found it sold for only 4 gold to a vendor. Doh! Not the reward you'd would have expected from beating such a hard dungeon.
Now we do know that pre-2.0 patch in Molten Core the loot tables were not totally random. Paladin loot didn't drop for Horde raids, and Shaman loot didn't drop for Alliance raids. So modifying the loot tables in function of the group going into an instance is technically feasible.
So my dream feature for today would be dungeon loot tables that takes into account group composition. If there are 2 plate wearers and 3 cloth wearers in the group, there should be no leather and mail drops, and there should be a 50% higher chance for cloth drops than for plate drops. It still leaves you with random chance, and the possibility that the drop isn't better than what you are already wearing. But at least it saves you from these annoying drops nobody could possibly use.