I enjoyed the somewhat more complicated narrative structure of this episode, switching between past and present stories and playing with POV. I like novels that do that, and I'm trying to learn how to write stories that do that as well. One reason why I think I liked it here is that it meant there was no A vs B plot; it was all one complicated story. My favorite A & B plot episodes are those where it's most obvious that A and B have strong thematic resonance (What if B led to A?).
[See the AV Club review & comments for more discussion of the episode's structure and the inspiration for it from some House episodes Liz Friedman also worked on. I am not familiar with them myself.]
Someone correct me if I'm wrong, but I think this was the first time we saw flashbacks to previous episodes (I'm not counting any "previously on Elementary" snippets). The first two flashbacks illustrated references by the lawyer to Sherlock breaking into Victor Nardin's apartment and the funeral home; the third illustrated her comment that officers believed Sherlock had taught Watson how to pick locks too. [gifset not by me] And what an intriguing selection of episodes they were, in the context of "Tremors" and the upcoming mid-season stunt development:
"One Way to Get Off" - Watson and Sherlock at odds but finding common ground in the work (at Watson's instigation), Watson obtaining a packet of letters from (we know now) Moriarty, and Sherlock telling Watson that Irene died.
"A Landmark Story" - Watson taking up the scalpel again (at Sherlock's instigation), Sherlock directly acknowledging that he's changed by his association with Watson, Sherlock ditching Watson again (albeit with a letter of explanation), and the reintroduction of Moriarty through Moran, Douglas, and a phone call.
"We Are Everyone" - Sherlock bursting with pride over Watson's initiative in learning to pick pockets, Bell and Gregson accepting/overlooking the requirement of "getting creative" with how the watch was obtained, Sherlock and Watson having potentially incompatible needs for outside interaction, and a letter from Moriarty.
In each of these episodes, Watson has relatively few qualms about Sherlock's methods: she voices concerns in the first two but her anxiety is more over getting caught than the right or wrong of the actions. In the third instance, she takes the initiative to employ his methods herself, and she's proud of the accomplishment. "One Way to Get Off" is full of issues related to murky ethics in police investigations and Gregson's complicity or willful blindness to them; there's a bit of a nod to that in "A Landmark Story" when Gregson and Sherlock speak after Douglas has been killed. In "We Are Everyone" it may be that Watson didn't think through the implications of stealing the watch from a police procedure standpoint as it's Sherlock who outlines what has to happen to use the evidence. But neither Bell nor Gregson comment on it one way or the other, and the episode concludes with the criminal caught, period.
On a communication level, Sherlock and Watson are about halfway through her contract in "One Way to Get Off" and he's upset by her prying into his life; he said he'd abide by the letter of the contract but any goodwill between them is gone. By the end he accepted her methods and shared a painful personal secret. They're mostly operating under the structure provided by the companion-client relationship. In "A Landmark Story," they're both rather blunt and uninhibited with each other: Watson blurts out a wish for a drink, startling them both, and he not only acknowledges his lack of remorse during "M" — after she brings it up —but gives her credit for why he's different now and leaves a note explaining his actions when he left her to confront Douglas. She's no longer his sober companion, and they're still perhaps in the honeymoon phase of their new partnership, not yet rubbing the other in irritating ways.
In "We Are Everyone," she's rather uncharacteristically open about her uncertainty with her work-life balance and is comfortable discussing her online dating venture despite his scorn. He's open about his preferences regarding socializing with others but hides the receipt of Moriarty's letter in plain sight. They're both a bit lost in determining how their lives will mesh now and into the future, given the explicit and implicit boundaries surrounding and dividing them, some conscious and some unconscious. They're stumbling in the dark, I think, and aren't sure where they want to go or how to get there. They're starting to notice that they might not even be trying to get to the same place. By "Tremors" it's fairly clear they're dangerously out of sync. It occurs to me that it must be Watson's turn soon to stumble because of the fault lines in their partnership; the danger can't be his alone.
I assume the Moriarty references are more to foreshadow her upcoming return and themes related to unreliable narration rather than to explicitly link any of the plot elements of "Tremors" with whatever happens later. Each of the references from the previous episodes took the form of indirect and deceptive communications: faked letters & faked death, faked phone call, and who knows what that last one is/will turn out to be but I assume it's at least 99.9% manipulation. In the first case Watson gets the letters and after a little deliberation returns them to Sherlock, unread. In the second, they're together when Moriarty calls and neither of them hides anything IMLTHO. And in the third, I assume Sherlock is still hiding the existence of the letter(s) from Watson. What might be faked or hidden once they meet her again face to face?