Guy Ritchie has done meticulous research into the details of the cannon, and many of the things critics complain of or scoff at have a firm basis. For example, for all the carping of Holmes participating in a bare knuckles boxing match, Holmes is described as a gifted bare-knuckles boxer in the Sign of Four (indeed, much of the characterization of the characters relies on facts establish in The Sign of Four, and seems to take place chronologically between The Sign of Four and The Valley of Fear). In a few lines of dialog, they reference that Irene Adler is originally from New Jersey and explain away the husband whom she married at the end of A Scandal In Bohemia to explain why she is available again and up to her old tricks. Where he departs, it appears to be a deliberate departure to give things a 21st Century sensibility. But, accepting this as a reinvention no more heretical than Basil Rathbone's Holmes fighting Nazis, it is quite enjoyable.
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What the movie captures best, and where it most "humanizes" Holmes to the lament of critics raised on the aforementioned television and movie adaptations, is Holmes' mental restlessness and "bohemian" lifestyle when not engaged in a case. This is straight cannon, and I think captured very well. The challenge is to visually show something that Watson describes with words in his framing story narratives in the cannon. Thus, we see a seedy Holmes lounging about engaging in various experiments, often half completed as Holmes loses interest. We see Holmes diverting himself with alcohol and with unnamed medical drugs (at one point Watson says "you know what you're drinking is meant for eye surgery" which is most likely a tincture of cocaine). We should recall that, in the stories, especially the earlier ones in which time frame this appears set, Holmes is known to take cocaine and opium when between cases.
What I found most effective actually was the sort of mental noise that surrounds Robert Downey, Jr. when he is sitting at rest, restless waiting. You can see his brain picking up all these little details of conversation and observation but, without some practical goal driving him, it is just noise that he soothes with some meaningless distraction. There is also the device of Holmes considering mentally, in zero elapsed time, how he will use physical violence against an opponent. While a number of critics lamented this, being all delicate about the violence and complaining about the "action adventure hero" meme, it shows Holmes for what he was in the stories: a scientific fighter who uses his brains and energy to overcome opponents quickly and efficiently.
To take things in turn, I think there are three things that critics object to here under the "action adventure" label. The first is the action itself, which consists of Holmes rushing about in disguises, consorting with underworld informants, and doing battle with his opponents. All three, of course, have solid basis in the stories. Holmes is a master of disguise, with a wealth of informants in London's underworld. What is different is that in the stories, the action usually takes place off stage, with Holmes informing Watson of what he accomplished. But Holmes and Watson have their share of "on stage" fights and gun battles as well (indeed, Holmes explains his triumph over Moriarty to Watson in The Adventure of the Empty House by noting his knowledge of "baritsu, or the Japanese system of wrestling, which has more than once been very useful to me.")
Certainly these don't have the prominence they have in a modern movie. But what makes Holmes unique is that the violence is not violence for its own sake. It is guided by intellect and focus. He is a "scientific" fighter, often setting complex moves in place to bring down his opponent or pop up in a disguise pulled together on the run. He employs the "Holmesian method" throughout, gathering minute clues and astounding his friends and opponents by his ability to deduce things from the smallest hints and samples. In one scene, after being taken blindfolded to the secret location of a mysterious order, Holmes explains exactly where he is and the identity of his mysterious host from his detailed knowledge of London and the smells and sounds he encountered during the carriage ride. "The only mystery," Downey concludes in Holmes' fashion, "is why you troubled to blindfold me at all?"
I'll add the Holmes' final denouement with the villain Lord Blackwood, while held on the dizzying heights of London Bridge under construction, is also classic Holmes, including his vain and egocentric love for explaining to the helpless villain how Holmes outsmarted him.
The second complaint is for the fancy devices and the device of the villain claiming all manner of supernatural powers. This invariably invites comparison to Ritchie's Wild Wild West movie. Again, however, there is plenty of precedent in the cannon. The Hound of the Baskervilles most famously centers on claims of the supernatural, to which Holmes insists on applying his scientific methods. And, on occasion, Doyle employs a convenient new invention or technology. In The Adventure of the Empty House, for example, Tiger Moran uses an air gun of his invention that allows him to shoot silently (and is foiled when Holmes and Watson leap on him and overpower him in "action hero" fashion).
Again, these things do not receive the prominence or focus that they do in the movie. The movie is an action adventure drama, and the devices used are very steampunk-esque. But Ritchie is not pulling things out of the air or imputing things to the Holmesian universe that are foreign. It is a 21st Century reimagining of the character with grounding in the cannon, emphasizing some elements over others but still staying closely enough, in my opinion at least, to be properly considered faithful to the spirit and intent of the original. Nothing is more quintessentially Holmes than the discussion between Holmes and Watson after Blackwood's resurrection. Watson recounts mysterious things he saw in India in the service and asks Holmes if a supernatural explanation may not be warranted. Holmes responds "I do not reject a supernatural explanation, what I object to is theorizing without facts. Do that, and you begin to twist facts to match theories rather than shape theories to match facts."
Finally, there is the relationship between Holmes and Watson and Mary and Holmes and Irene Adler. Certainly Irene Adler has been updated to some degree, although again the roots of the character are based in what little we actually know of her from the cannon. The business in the ads of Holmes naked in the hotel room is not at all representative of the relationship between Holmes and Adler in the movie -- which is far more rivalrous than sexual. Yes, they are clearly interested in each other, but it is played very chaste and Victorian, with banter and declaration and not a lot of physical action. It is clear that Holmes finds Adler alluring because of her intellect and her ability to get the better of him on occasion. But this is a relationship of equals and one taken seriously and not one played for the joke of the superior woman putting the man in his place. There is an intelligence about their characters and interactions as well as a charming honesty between the two of them that only occassionally veers too close to Nick and Nora Charles territory.
More interesting and central to the movie is the Holmes/Watson/Mary relationship. Happily, this is not played as some sort of sexual bond between Holmes and Watson. Yes, Holmes starts by trying to sabotage Watson's relationship with Mary. But it is clear that what drives him is his fear of losing Watson as his right hand man. Watson himself anticipates that when he marries and moves out of 221B, he will no longer go adventuring with Holmes and will settle into the role of quiet family man with a private medical practice. It is this loss, the loss of a trusted companion with whom Holmes can share his adventures, that makes Holmes want to break up the impending marriage.
At bottom, Holmes without Watson is lonely. No one else in Holmes' universe appreciates him in the same way -- as a friend and equal rather than as a legend or as an intolerable pig. Without Watson, Holmes fears he will drift off on a tide of mental idleness and dissolution. Watson is his anchor to the world and to the humanity which Holmes arrogantly regards as inferior but whom he desperately needs. We see this when Holmes is alone, between cases, trying to stimulate himself and looking dissolute. When Watson is present he becomes focused again, engaged in real conversation.
The conflict between Holmes and Mary is nicely played, as is the resolution. When Watson is injured, Holmes (for various reasons, now a fugitive from the law) visits the injured Watson disguised as a treating physician. Mary comes in, she makes it clear she knows it is Holmes and that she respects the relationship he and Watson share. It is brief, intelligent, and not overplayed (as such scenes all too often are). Afterwords, Watson never talks of this being his "last case" with Holmes and Holmes becomes quite reconciled to the coming marriage.
Holmes and Watson are clearly a team-- albeit with Holmes as the dominant partner as he was in the stories -- and they share a deep affection for one another. Watson displays his medical skills, and is as useful in a fight and with a gun as he was in the cannon. Yes, they are more equal and familiar with each other than the Victorian gentlemen of Doyle's day. But the business of Watson slapping Holmes in the carriage which one sees in the advertisements is, as with the Holmes handcuffed and naked scene, entirely atypical of the interactions between the two. These are two familiar comrades, with one about to get married and move on to a new stage in life and another feeling the anxiety that comes when something safe and familiar suddenly changes. Critics who object to this as being too much like a modern buddies action movie need to review the stories to see that all these elements are there in the original, especially for anyone familiar with the conventions of Victorian literature.
As an aside, I have to say that the advertising has really done the movie a disservice. It plays this as a conventional action adventure story with the notion that the old "stodgy" Holmes has been replaced by something goofier and more physical -- a representation fed by the critics who appear utterly unfamiliar with the cannon. Ignore this.
The bottom line is that the movie certainly contains 21st century elements. But it is a product that remains faithful to the spirit of the original. As a movie, I think it works very well. None of this JJ Abrams crap of nonstop action and bizarre camera angles with never an opportunity to get to know the characters. Amidst the steampunk SF elements and the action sequences we have interludes of genuine character development. It is a mark of how well the movie works that the characters seem both fresh and still faithful to the original archetypes.