The difficulty with any adaptation of the Sherlock Holmes canon is that you are dealing with a fanbase in existence for 125 years, spanning generations. Naturally these people like strict adherence to source material; however at the same time you can only read or watch the same events take place so many times before you hunger for something new. This is why I enjoyed Sherlock Holmes: A Game Of Shadows, an adaptation that manages to be fresh and surprising.
The events of the short story "The Final Problem" (in a nutshell: Holmes sets out to stop the plans of Professor James Moriarty, succeeds but has to escape to Switzerland and gets 'killed' at Reichenbach Falls by Moriarty, who loses his own life in the process) are kept to in broad strokes, which will please the die-hard fans. However, turning a short story into a 129-minute film allows for a lot of additional material along the way, including slow-motion fight scenes, some genuine surprises, a shot of Stephen Fry's nearly-naked body, and more slow-motion fight scenes. Interesting, they've created something new by borrowing from a variety of sources (hey, it worked for Star Wars).
The slow-motion fight scenes are a director's choice that he chose to bring in from the last film. The idea of Moriarty being behind World War I two decades early is straight out of the film version of The League Of Extraordinary Gentlemen. And I thought I detected a hint of Lemony Snicket in the narration by Jude Law at the end (which is inevitable when you get the same actor to do it but it was a nice touch all the same).
The thing on which Holmes fans will be judging this film is "how good is Moriarty?" Judging from all of the incarnations I've seen so far, in my opinion Jared Harris' portrayal is the best of the lot. He's young enough that you can buy that he's a genius professor but old enough so that you can tell he's got some experience in the darker sides of the world in a way that Andrew Scott's (Sherlock) Moriarty doesn't, for example. He's a former pugilist, which puts him on even physical footing with Holmes and he has the right amount of steel lying just below the illusion of respectability. I also thought that, world ending schemes aside, his version was more realistic than the others. He's not entirely infallible, which makes it easier to see how Holmes could hope to best him in a fair fight (which, admittedly, the final fight isn't).