Everything all just came perfectly together.  The writing knows to give out just the right amount of information to make the story engaging but not so much information that it isn't scary or mysterious.  The acting is incredible and truly top-notch.  Speaking in perfect sync like that must have been so hard but the actors make it seem natural and easy.  The effects aren't overwhelmingly good or anything but that just makes Midnight all the more impressive - it's clearly an episode with a limited budget and basically 2 sets, yet production does so much with so little.  Easily one of my favourite stories in all of Doctor Who.