Moral Event Horizon: Hardly any definite ones in the main story, but the flashback about Agha Mohammad Khan Qajar razing Tiflis and having his army kill most men and rape and cripple all the women they could get their hands off probably counts.
Arguably, Doctor McNeal knowingly arranging the destruction of the Russian embassy and Samson Khan beating his own daughter (probably to death) in a blind rage after sending away her fiance to likely death count as well (though in Samson's defense he was both really provoked before crossing the horizon and clearly remorseful afterwards).
One-Scene Wonder: As mentioned, there are plenty of interesting, well-written characters like Senkovsky or Yermolov who only appear once or twice in the book (though Yermolov has a slightly bigger role in another one of Tynyanov's historical novels).
Unintentionally Sympathetic: Tynyanov's psychological writing style and the gray morality of the novel lend themselves to fairly sympathetic interpretations of characters who initially seem like they might be simple strawmen and/or are traditionally treated rather negatively by historical memory and historiographical tradition. Bulgarin (traditionally vilified in Russian literary history courtesy of Pushkin) might be this, though he is clearly somewhat sympathetic from Griboyedov's point of view as well. General Paskevich might just be the ultimate example of this though, as despite initially seeming like an idiot and a General Failure, he is later revealed to be a competent, intelligent (if not Book Smart) commander who was thrust into a position of authority and then routinely mocked and slandered by everyone for not being a genius (and despite still being objectively much more successful than Yermolov, "a great general who never had a single victory", and clearly not just because of his supposedly hypercompetent subordinates - it is mentioned that being an admittedly lousy strategist actually made him a master of the Indy Ploy, and beside that he is good with things like tactics and organization). In fact, his seeming Plague of Good Fortune makes things even worse, as no matter what he achieves, people just call him incredibly lucky rather than talented in any way. Who wouldn't become a resenter?
Values Dissonance: Griboyedov and other characters have very authentic early 19th century opinions about women and savages (though Griboyedov is not so sure about the savages, having far fonder memories of Tbilisi than of Moscow or St. Petersburg).