I do not know what it was but prior to watching this recently I did not know why this one was as popular as it was. It was a good story but not a great.
This time however I really enjoyed this. The production department have done an amazing job in realising the era, the monsters are creepy, Captain Jack is a welcome edition to the TARDIS, the performances from all of the guest cast are mesmerising and the story is so gripping. Overall a classic. 10/10