For me it's my friends(one of them) leaving without completing it. Also they complain that one of my friends doesn't have G-horn so...yeah. They complain alot. x.x So they leave soon as we get to Skolas. They also don't like the Cabal area where you have to dismantle mines the Cabal in there is too difficult. It takes us 3-5 tries to finish it. Its not easy the Cabal are the most hated enemy for us in Destiny.