There are no actual plot spoilers in the following -- but then, there's not much plot, though a lot of running about, and a whole lot more of talking-talking-talking.
Who is Mary?
We've had centuries of adventure tales in which generally there is no knowing who are the wives, girlfriends, mothers, and other female accessories of the male principals -- because evidently the composers and the audience didn't find these women of any value beyond, possibly reward for Heroes' heroism and, presumably, make dinner. We're so used to this we don't notice -- we don't question even to ourselves -- what we don't know. Or so Moffat and Gattis evidently believe.
In "The Empty Hearse" we don't notice that all we know of Mary is that Watson fell in love with her, she loves Watson too, and they want to get married. She's fulfilling the non-expectation of agency and identity of the typical male protagonist's accessory. O! lucky woman! -- she will likely die soon and / or be raped as our protagonist's plot and character motivation. Beyond that, all that matters is whether Sherlock accepts and / or likes her, and he seems to.
Which is more than enough after centuries of conditioning -- until Sherlock's scan reveals "Liar."
Then we forget again, because in "The Sign of the Three Sherlock" reveals that Mary's pregnant, which apparently explains "Liar," i.e. she hasn't told Watson yet. All innocuous, until the third episode, "His Last Vow," in which it all changes.
This does not obviate that, for this viewer, season three was seriously disappointing. In fact "The Sign of the Three' was such a tedious talking head sort of episode that I spent less time wathcing it than I spent sort-of listening to it while peeling, chopping, and otherwise doing dinner preparation. Sherlock performing his memory palace and scanner shenanigans are something viewers look forward to. But this time around the shenanigans had me impatient for it all to be all over. This bode badly for "His Last Vow."
Initially, "His Last Vow" seemed to promise full redemption for the previous wretched family - fannish dramaration that the first two episodes had been. We had a case! A case that we can accept matters to national and even global security!
And then ... and then ... and then ... as the old song says, "Along came Jones." Or rather, along comes a deluge of cheap-shot sociopaths -- o goody, Mary's one too! We are all one big unhappy family of sociopaths -- Sherlock's parents, Mycroft, Watson, Mary and Sherlock. Merry Christmas to us all every one.
There is a single exception to this disappointment, which, if more thought and time were given to it, would provide a lot more interest than tossing yet another sociopath in the mix: we learn something about the Holmes brothers' mother that is very cool. Will we learn something equally interesting and explanatory about their father next season? Not a hint of that anywhere. So this bit was probably just a toss-off.
But now goodbye forever to Sherlock because he sacrificed himself for Watson's happiness. Which we all know sociopaths as a matter of course care deeply about other people and their well-being.
This is why Sherlock can blithely lie to, manipulate and fake romantic-sexual love with an innocent woman to get what he wants -- and care not this is a rotten thing to do to another person.
In the meantime, pregnant Mary .... The upshot of her plot shot is -- Sherlock's banished to sure death in 6 months. But along comes Another Reverse! Mycroft calls back Sherlock because something really really REALLY huge is afoot.
This hugeness is Another Reverse! Guess who else has returned from the dead? The creepiest most boring psychopath of them all. Ho-hum. Forget canon. Forget everything when everybody is so Super they are predictable, uninteresting, Jacks in the Box, not a corpse in the body bag.
Telenovela family dramaration is the easiest formulaic stuff to write. Creating, foregrounding and centering intricate, interesting case episodes, with sufficient action and tension, that might even be seasonal arcs, in which the personal relationships are essential, but not the focus, is about the most difficult television writing to do. Moffat and Gattis aren't up to it. They have confused cheap, pandering with creativity and smart writing.
It's unlikely I'll be watching next year.