These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In episode 8, Richelieu tells the murdererous thug Labarge, "Your reputation for violence is established. Now put it to use." Labarge is played by sometime soccer thug turned hard-guy actor, Vinnie Jones.
Base Breaker: Monsieur Bonacieux's suicide attempt. The reaction from the fandom seems to be either sympathy for him, hatred that he didn't die so D'Artagnan and Constance can be together, or disgust that he blackmailed Constance into staying with him by threatening to kill himself if she ever left him again.
Since Peter Capaldi was cast as the Doctor, the poisoning scene at the end of the first episode now looks like a deliberate reference to Fedorin's death in "The Enemy Of The World": an ambitious political villain who happens to look just like the Doctor disposes of an unsuccessful minion with poisoned wine.
Luke Pasqualino played Freddie in Skins. Freddie and his friends Cook and JJ called themselves the Three Musketeers. Might be Actor Allusion.
The Man in the Iron Mask, an adaptation of the Musketeers' later adventures, also took the liberty of one of them having an affair with the Queen - namely D'Ártagnan.
In First Doctor episode The Massacre the Abbot of Amboise, another 17th century French clergyman, is played by William Hartnell.
Ho Yay: When Porthos is hurt in "Commodities", Aramis almost loses it at Athos for being willing to risk Porthos's life just so they can get a couple miles down the road and Athos doesn't have to reveal his former household.
In "The Homecoming," Aramis again loses it, this time at d'Artagnan for questioning whether Porthos was guilty of manslaughter.
I Knew It/The Untwist: Anne being pregnant, after the events of "Knight Takes Queen", was the exact opposite of surprising.
Moral Event Horizon: For the Cardinal and Milady in "A Rebellious Woman", when they frame an innocent woman for witchcraft and lesbianism and try to have her burned at the stake because the Cardinal wants her money to renew the Navy and because it may help him become the next Pope.
In-universe, this occurs much earlier in the series when the Musketeers find out that the King, Cardinal Richelieu and Captain Treville himself sent twenty soldiers on a supposed training exercise, only to be slaughtered by the Duke of Savoy and his men, all to protect the King's sister, who is spying on her husband the Duke for France. In Treville's defense, he didn't know about the massacre until after it happened. Aramis - one of two survivors of the attack - is still shaken by what happened.
Richelieu definitely crosses it in 1.09 by trying to have Queen Anne assassinated for no discernible reason other than the King's drunken lamentations of her not being fertile enough to give him a son and heir.
Mind you he does reveal his reason in the next episode he believed the queen to be infertile, and feared that if the king was to die, France would be plunged into civil war.
Older Than They Think: Some reviewers considered the depiction of the central characters' sex lives, in particular D'Artagnan's relationship with Constance being adulturous and the other Musketeers having sex with rich middle-aged women for money, to be gratuitously Darker and Edgier. In fact, those elements are included in Dumas's original novel, but were left out of many earlier Bowdlerised translations and adaptations.