It Was His Sled: The dramatic revelation, two-thirds of the way through, that the demmed idiot Sir Percy Blakeney is the Scarlet Pimpernel, to the point that no adaptations tries to hide this.
Unintentionally Sympathetic: Modern readers would find the Jew who aids Percy despite the potential danger to himself and his family much more heroic than Orczy would probably have intended.
Similarly, the depiction of the Jewish prisoner or rather, Sir Percy's depiction of the Jewish prisoner might be read with some discomfort by modern readers. Note that Percy is deliberately playing to the Frenchmen's prejudices.
There's also perhaps a touch of Reality Subtext here: at the time the Baroness wrote the novel France was still hashing out the Dreyfus Affair, which wouldn't be satisfactorily resolved until 1906.
Values Resonance: Despite the above issues, in a broad sense, depicting rescuing persecuted refugees and helping them resettle in a new country seems even more relevant amid a number of serious refugee crises in the twenty-first century.
Sir Percy Hits Back reveals that Chauvelin has an 18-year-old daughter. Count how many adaptations make him an ex-suitor or love interest for the 25-year-old Marguerite. Not a lot of hindsight is even required here, since almost the first thing the original novel says about him (even before the first "fox-like") is that he's "nearer forty than thirty".
In The Life and Exploits of the Scarlet Pimpernel, written by Baroness Orczy's son, Sir Andrew recalls the day Blakeney explained his reasoning behind adopting the scarlet pimpernel motif and his m.o. of leaving his calling card whenever he made a rescue:
1937's The Return of the Scarlet Pimpernel, Maximilien Robespierre is the head of the Committee that rules France. Responsible for the Reign of Terror, Robespierre sentences countless people to the guillotine, enemies of the state and rivals alike. Plotting his own coup and a purge of any potential rivals, Robespierre also sends his agent Chauvelin to kidnap the Pimpernel, Percy Blakeney's wife Marguerite, while she is pregnant, deciding to have her sent to the guillotine regardless of her value as a hostage before providing twenty more names to be arrested and executed.
Musical: Citizen Chauvelin is a ruthless agent of The French Revolution who delights in sentencing people to the guillotine, opening the musical with the execution of the good-hearted Marquis de St. Cyr. Hunting others to send to their beheadings, Chauvelin also blackmails his former lover Marguerite by threatening to ruin her marriage to Percy Blakeney, also using the threat of her brother's execution to control her. Intending on killing them all when he realizes Percy is the heroic Scarlet Pimpernel, Chauvelin embraces nothing so much as spite and rage in his attempt to purge all he despises.
The Woobie: The 1999 TV version has Tony Dewhurst, played by Jamie Bamber in the middle of the Professional Woobie stage of his career. Such a Cutie and killed off so quickly, becoming the first of many of Bamber's characters to bite the dust (adding insult to injury is that Dewhurst did not die in the books the series is based on). TV Tropes send the guy a hug.
, which wouldn't be satisfactorily resolved until 1906.