"To Sherlock Holmes, she is always the woman." (A Scandal in Bohemia)
I haven't read every Sherlock Holmes story, and I've certainly not seen every iteration of the famous detective ever made. I do think it's interesting, however, that so many filmed versions focus on recurring characters that don't have as much of a role in the original stories. Moriarty isn't nearly the player most seem to think; if anything, he's actually a rather weak nemesis. Irene Adler, to my memory, only actually appears in one story.
While "A Scandal in Bohemia" is surely a classic, and shows one of the few times that Sherlock Holmes is outsmarted by anyone, it doesn't do much with Irene beyond making her clever, and sentimental. Updating her to a gay (?) dominatrix works with some of the original intent--Sherlock does take the foundational story of "Bohemia" as inspiration while expanding it significantly--but I'm not sure I ever read Irene as the arrogant woman portrayed this week.
I don't have a problem with this, though. Overall, I think this was a very strong episode. It didn't meet the same tension and shock that "The Great Game" provided, but it didn't always need to. I enjoyed the Coventry twist and Mycroft's decisions to protect his brother. I enjoyed seeing Sherlock thrown off his game, becoming even more inscrutable. I loved Sherlock and John's dedication to Mrs. Hudson--even when, in Sherlock's case, he didn't demonstrate it the way "normal" people would.
I do find it fascinating that so many people are drawn to a man who seems, on the surface, to take no interest in individual people. John, Mycroft, Lestrade, Molly (poor Molly), and Mrs Hudson all show the utmost dedication and loyalty to a man who, if they didn't know him any better, wouldn't ever seem to repay in kind. Even Irene Adler, a self-proclaimed lesbian and dedicated con-woman, falls for him.
It isn't often that Sherlock seems human at all, but this episode builds on the end of "The Great Game" in a subtle but engaging way. Moriarty's first exit from the pool at the end of that episode gave us a chance to view Sherlock Holmes as human, if only for a moment. All the tension slowly leaks away as Sherlock gets the bomb off of Watson, then prattles on -- and who would ever expect Sherlock Holmes to prattle? -- about John's actions and how they were "good." He was at a loss for words, a highly unusual state for a man who would "outlive God trying to have the last word." Sherlock, for once, is not infallible. Beyond that, he shows in his own way that he truly cares for John.
Moments like these are rare, and subtle, but absolutely worthwhile and well done. Consider Sherlock's apology to Molly, the way he scolds Mycroft for telling off Mrs Hudson, the slump he seems to fall into over Irene's apparent death. It is for moments like this that Sherlock's friends put up with his idiosyncrasies and rudeness. It is for moments like this that the audience of the show can find a human moment beneath all the cleverness and wit.
If I had only one criticism of this episode, it would be the fact that some moments are over-explained. Sherlock is frequently subtle; it would take a Doyle expert to pick up every small in-joke and canon reference. Even the protagonist's drug addiction is only hinted at; a "danger night" after losing Irene was still only briefly touched. However, there were a few moments that lost that subtle edge. I'm thinking of Irene's explanation of the combination after we'd already seen Sherlock put in the numbers (32, 24, 34) and of course, the final scene. I take the greatest issue with the latter. Any show does its best by leaving the audience wanting more--and a little mystery certainly never hurt a mystery show.
ETA: There are a few other thoughts I've had about this episode, but one I feel I need to mention right away is Mycroft's assistant Altheia (though I believe this is not her real name) and her apparent involvement with Irene. Is this a personal connection--does Irene simply know what Althea likes? Is she compromised altogether? If she's working with Irene, does this mean she could be working with Moriarty as well? What does Mycroft know of all this?