Well.. here are my reasons...
First: harvest node, if i run quest with other player only one of us will get the node. if we separate we each get our own node.
Solution: Change material node to GW2 style, where every player loot their own node.
Second: treasure chest, weather it is in dungeon or group.. it is always annoying that other get treasure chest while you don't...
Solution: each treasure chest provide loot for every group member.. so if someone open a chest everyone will get to loot their share.
Third: There is absolutely no reward for running in group....
Solution: give a party bonus of 5% exp for running with a group member in range.