The ones involving the Prison of Elders challenge mode, the Raid, and that Crucible quest are ones I question if I'll ever complete. I've only ever played and made significant progress in the Raid twice, and [i]both[/i] those times I had to leave halfway through to do work in the real world. It's not easy to find people on the lfg sites to play with if you don't already have top-tier gear--which you can't get without doing the Raid in the first place.