Steady pace, great characterisation on all the cast, not just regulars or the guests. But both Mark Eden (Polo) and Hartnell deserve special mentions for great performances. Says a lot that I enjoy the story so much with just audio, script, and telesnaps.
Also watched Nigel Kneale's The Stone Tape today, which was also pretty darned cool. The characters were three-dimensional and well realised, and the story was slow but steady. And at times really creepy. Easy to see how it helped inspire Spielberg and Hooper on Poltergeist, and Carpenter on Prince of Darkness (though Kneale thought that film rubbish).