This characterization of Sherlock consistently pisses me off. It's the trope of the brilliant man, socially inept, pushed to a ludicrous extreme, and I find that trope offensive to begin with. Sherlock as portrayed in this show gets away with being an asshole to everyone, and it is no more obvious than in his return from the dead in series 3, and how he goes about 'telling' John.
In the original text, he shows up - sure, it's always gonna be a shock - and apologizes. Doesn't engage in a 'funny' game of 'surprising' Watson in public. Doesn't assume Watson has just gone on in exactly the same way with his life as when it included Sherlock. Shows NO awareness of John's grief at all. I hate it. It's cruel. John should beat the shit out of him - that's a realistic response, and I was pleased to see it.
Frankly, Sherlock's conduct undermines the emotion (on his side) of his earlier death in the first place.And then he does it again, basically - provokes a display of genuine emotion and caring from John (in the subway car at the end of episode 1 of series 3) and LAUGHS at him.
Now, in Episode 1 of Series 4, Mary's past catches up with her, she flees, John and Sherlock catch up with her, Sherlock promises his protection, and she ultimately dies. Her dying words are full of gratitude to John for giving her a life she was actually proud of, etc. So, from one angle, Sherlock's hubris kills her.
Plot-wise, the setup for her danger in the first place, and the idiocy with the usb drive - they're just stupid plot points. Keep incriminating info on your body at all times, while running around being some sort of special ops merc team? Jesus, that's dumb.
Back to Mary. You can read her death as - as Mary put it - the logical consequence of her actions in her life, her past choices. She clearly chooses to face a bullet rather than have a comparative innocent (in the situation) face that bullet for her, face those consequences, of her actions. So, from that angle, her resulting death is part of the exercise of her autonomy, which I really like.
But I think the trappings of her making a dying declaration in John's arms, that the life she had with him was the only one she was proud of, or whatever, will make it too easy to have her be just another 'fridged' wife instead of an autonomous actor making the choice to face the fallout from her prior life.
Using the death of his wife as a trigger to finally let out John's entirely justifiable anger at Sherlock as a real consequence of Sherlock's hubris makes sense in context. I'm pissed that the wife has to die, but Mary isn't present canonically anyway, so, eliminating her from the TV series makes some sense - kind of like the Hornblower TV series needing to shift emphasis from the Hornblower/Archie Kennedy friendship to the canonical Hornblower/Bush friendship - because it brings the series further into conformity with canonical pairings.
Here's my deal: if Series 4 doesn't provide some emotional resolution - and there's a hint in episode 1 that it could - of how terribly Sherlock has treated John, than I'll be very glad if this should be the final series, because their relationship is horribly damaging, and John Watson deserves better than that.
Canonically, Sherlock Holmes values and loves Dr. Watson immensely. They socialize together. Holmes knows he can rely on Watson to be steadfast support, to help him in a pinch, to cover for him and delay the murderer in the hallway whilst he hurries on to the drawing room to stop the murder and reveal all. He values Watson's medical training. He encourages Watson to apply Holmes' own methods, and critiques his results. He seems to regard Watson as his equal, regardless of Watson's lack of his own particular brilliance in his chosen field. There is always a deep affection, canonically, between them.
But this version of Sherlock - I can't think of why Watson would or should care about him.