Title spells it out pretty well...
Some quests seem to have diminished XP rewards based on the idea that the loot will draw people in.
As vale clearly illustrates, having loot people want only encourages CAP groups of running it. Making the XP the quest may or may not give out drop to 0 since the massive over level group farming the loot.
So, quests that drop desirable loot are skipped due to the low(ish) XP, and difficulty, out leveled, and subsequently farmed on "easy mode" to ensure loot is gained.
Quests that drop massive XP are farmed, and solo farmed at that to maximize character gain in the race back to 20+. Any loot contained will be picked up at cap or way over quest level to ensure the next trip to 20 you have the BEST items for the level.
The result is people racing to cap skip MANY/MOST quests because the XP is what we're after, not the loot. The loot quests are saved till we can farm them, and we don't group them unless 100% needed.
Is there a mechanic to be added to 'push' or 'favor' grouping while not gutting the current play styles?
1) XP bonus based on "player" membership? +5% for 2 man group +10% for 4 player group + 15% for FULL group.
2) Increase the XP in some quests that are under played to encourage play. (Even if they have good loot.)
3) Start selling rouges for plat sub 20. (You already sell 21 level rogues) Or modify elite traps to not be a pass/fail condition of epic quests.
The current group incentives are just more rolls at loot. Who cares... if the current method is cap farm for loot.
The current group requirements are simply have a trapper for certain quests, or just skip them.
The current run this quest incentives seems to be loot + XP. But loot is irrelevant because we'll bypass it and farm it capped out when it is EASY to get for the next life.