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.
Magnificent Bastard: Richelieu, in spite of opposing the heroes through most of the book and being quite a ruthless bastard, ultimately comes across as strangely likable to both d'Artagnan and the readers. By the end of the book, their mutual respect is so great that Richelieu gives d'Artagnan a promotion. In later books, the Musketeers look back fondly on Richelieu as a worthy adversary with some measure of greatness. Probably inherited from the original. In the second book, Athos and D'Artagnan both mildly concede that they may have been on the wrong side, considering how badly Louis fared. Mazarin thinks he's this, but his greed and miserliness holds him back.
The point could be made that Richelieu never saw them as actual enemies, and saw them more as "Those damnable kids" since when he *does* think on them, he wishes he could win them to his side, rather then killing them outright. He merely wants to rule France through the king as his puppet, and seems to admire their boldness, even if it does stymy his plans. He even takes them as his guards in the dark of night, when they were all on the field of battle against a city full of Puritan dissenters.
You have to hand it to Milady as well - locked in a small room and guarded by someone specifically chosen as loyal and impossible to seduce, she turns him into her very means of escape and of assassinating the Duke of Buckingham.
Debatable: His illegitimate birth haunts him in social scenes, he hardly gets into duels, he pines over Louise de la Valliere only to lose her to the King, and in the end commits suicide by charging against the enemy.
Sequelitis: The sequels are not as widely known as the original.
Values Dissonance: Frequently Lampshaded by Dumas, as he often breaks the narrative to wryly note that his heroes' womanizing ways were just common practice in those days. Possibly actually meant to be a Take That against practices in his own time. This trope is notably averted when D'Artagnan seems just as troubled by Athos' murder confession as the reader is. The fact that he later turned out to be mistaken only complicates the matter further.
Villain Decay: Going along with Le Vicomte de Bragelonne's main theme of how the new generation is far too Romantic and not as adventurous as the previous one, the villains suffer as well. De Wardes (son of the man d'Artagnan wounded in the first book) stirs up some romantic tension and briefly duels two people, but beyond that, is a far cry from Milady or Mordaunt. Similarly, Colbert lacks the respective cunning and manipulation of Richelieu and Mazarin.