The plot of this episode was not very "elementary". There were not too many suspects to build much complexity. Moreover the conclusions drawn by Holmes were not very well justified. He built up his improbable hypothesis just on the basis of a "nail with a blackened tip" which was very unlikely of Sherlock Holmes.
What I admired in this episode was the nostalgia of Conan Doyle that was introduced. Agent Lestrade of Scotland Yard...221b Baker Street of London, the "sanctum sanctorum" of Sherlock Holmes...and Mycroft, Sherlock's brother with no ambition and energy. But the nostalgia will not be enough to survive the entire season if the successive episodes continue to be as dull and drab as this one.
Jonny Lee Miller and Lucy Liu were brilliant in their roles as usual, though Watson did not have any significant part to play in this episode.
This series had been as enticing as the BBC one and I earnestly hope that the writers improve their stories and maintain their level of excellence that they had portrayed in the former season.
I would give it a 7/10 just for its nostalgic element.