4. Moriarty
In the books, the professor only shows up in one story yet his wider infamy seems to stretch further. Moffatt and Gatiss pulled a masterstroke, first by having the shadowy figure pulling the strings of crime unseen and then, when he is introduced, by casting Andrew Scott. He is perfect as the arch enemy and his piercing, unblinking eyes ooze evil genius. His demeanour switches from calm, methodical gamesman enjoying the battle against Sherlock to mad, psychotic killer and back again in a flash. He also does a pretty natty line in clothing himself and the face offs between Moriarty and Sherlock are the most memorable moments of the series.
3. Writing
Tampering with such well known stories can be very dangerous but the writing team of Moffat, Gatiss and Thompson manage to update the stories without losing any of the charm and remaining faithful to the Conan Doyle originals. Notably, A Study In Pink manages to contemporise the forensic detail and curiosity of the first Holmes novel whilst retaining the mystery and drama that make the original story so interesting. There are some killer lines in the series (many reserved for Mrs Hudson) and, true, there are moments where the in references grate a little (Grimpen Minefield anyone?) but we can forgive them that, cant we?