I always explain boss encounters to new players. Hell, I spent most of yesterday joining session in progress as WHM in Amdapor Keep just so I can help groups that were abandoned by some impatient healer.

There were a lot of undergeared players there for the first time so I pushed them through to the end everytime even if I had to tank half the dungeon. I also gave pointers on how they can upgrade their gear to make their future runs smoother.