In this day and age, one does not expect to hear the sentence "Mr. Holmes, they were the footprints of a gigantic hound!" In this adaptation of the 1902 Conan Doyle story about a monstrous hound stalking the moors of Dartmoor, it is the phrasing that grabs the interest of the great detective rather than the contents of the sentence. It is this example that serves to underscore how great an adaptation it is of the original stories and also subverts your expectations formed by the existing source material. Writer Mark Gatiss takes a relic from the original novel (which I've referred to as the Sherlock Holmes equivalent of a money shot, in that it's what you expect to see in a Sherlock Holmes adaptation in the same way a money shot is used in pornography) and makes it essential to the solution of the mystery.
However, the actual problem that I had with the episode lies in the mystery itself. Although it had a lot of twists and turns, I'd worked out the overall solution to the mystery in a matter of ten minutes. Admittedly the motives and the means were somewhat beyond me at this point but the full solution came from information that we didn't possess (in simple terms, I had the how, but not the who or the why). If you can get past the roadblock of mentally screaming at Sherlock to think of the obvious solution, then the rest is a well-done horror story.
Holmes fans will notice that most of the names from The Hound of the Baskervilles are still there in some form, with the exception of Selden, the escaped convict from the local prison. Barrymore is now a major at the local military base and Stapleton is now a female scientist. Those expecting it will also notice the Grimpen Minefield, rather than the Grimpen Mire. The perpetrator and Holmes' client are completely new, albeit falling into the roles of existing characters.