There's a lovely callback to the show's first ever episode when Lestrade makes a small appearance at the end of The Final Problem.
When he catches one of his officers staring at Sherlock, Lestrade asks him why. "Well, he's a great man," the PC replies. "No, he's better than that," says Greg. "He's a good one."
Back in A Study in Pink, John asked Lestrade about Sherlock. At the time he said: "Sherlock Holmes is a great man. And one day, if we're very, very lucky, he might even be a good one."
The clear implication is that this day has finally come, and that Sherlock's journey from high-functioning sociopath to hero is now complete.
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