It just occurred to me that in each of the three series there has been an episode in which the villain was introduced in the last few moments - just when we think the episode might be over. It happens in The Blind Banker, The Hounds of Baskerville and The Empty Hearse.
I think it's safe to expect the same to happen in series 4 (not in the Christmas Special).
And, of course, there's the cliffhangers. There always has to be a cliffhanger.
Have you noticed other patterns throughout the series?