Since it was mentioned, Qarn and Cutter's Cry are basically the idiot killer dungeons. Qarn especially, but Cutter's Cry's first boss can be a nightmare with bad players. Qarn teaches you positioning mechanics (I think the final boss has more mechanics than anything save the HM Primals) and Cutter's Cry is when the baby gloves are taken off in difficulty.

I really wish they had been required. Instead they're optional and I know many people skip them.