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.
Outside of an actual Japanese song ("Miya sama"), the "three times three" wedding toast and a phrase supposedly from a children's game ("O ni bikkuri shakkuri to!"), the Japan of The Mikado bears no resemblance in the least to the Japan of real life — but then, it was never meant to.
The Yeomen of the Guard are not the Beefeaters who guard the Tower of London — those are the Yeomen Warders. The Yeoman Warders did not exist until 1548, and the Lieutenant of the Tower, Sir Richard Cholmondeley, mentioned in the lyrics, served from 1513 to 1524.
Crowning Music of Awesome: "Hail Poetry" from Pirates. Sort of an inverse BLAM. There's at least one great song in every show, but Pirates is particularly strong in this regard.
Fair for Its Day: The apparent anti-feminism in Princess Ida is nothing compared to the genuine Anti-Feminist jokes of its time. The Tennyson poem it's based on is also arguably worse in many respects than Gilbert's parody, since the Framing Story basically claims it's an incompetent attempt by feminists to rewrite history, which ends up showing that a woman's place is with her man. In Gilbert's version, the worst you get is some characters poking fun of women's education — before they get there, and all of whom think that educated women are fantastic once they meet them, skewering of some of the man-hating aspects of Ida's college, and a scene where book-learning meets reality, and the woman refuses to do surgery which she was taught to do from books alone. Plus, in Gilbert's other work, in Utopia, Limited, the Cambridge-educated Princess Zara never has this poked fun of, and is shown to be vastly more capable than most of the men, so it's not like he makes a habit of anti-Feminism.
In HMS Pinafore, just how old is Ralph Rackstraw supposed to be, anyway? Captain Corcoran has a grown, marriageable daughter. Buttercup claims (Unreliable Narrator, perhaps, to help sort out the mess?) to have raised both Rackstraw and Corcoran as babies and switched them. In fact, if one is paying attention, the three pairings at the end of Pinafore are all disturbing by modern standards. Ralph marries a woman literally young enough to be his daughter, Porter marries his own cousin, and Corcoran marries his own wet-nurse. Ick. At best, if we assume everyone started early, Josephine would be in her late teens, the Captain and Ralph in their mid-thirties and and Buttercup in her late forties. Nothing too bad...
If they're celebrating Frederick's birthday which happens to be in February wouldn't that mean all of the lovely seaside action in Pirates is taking place in the winter?
Ho Yay: The only woman Frederic has seen in all his years of piratical service is Ruth. Think about that for a second.
Magnum Opus Dissonance: Sullivan would much have preferred to be known as the composer of his serious classical music, especially the operaIvanhoe, agreeing with music critics who often took him to task for wasting time with comic operetta. (Although he certainly didn't mind that his collaborations with Gilbert were very lucrative.) Today, of course, his "serious" music is remembered only because it's by the composer of those comic operettas.
A notable Older Than Radio example is the "What, never?", "No, never", "What, never?" "Well, hardly ever." exchange from H.M.S Pinafore. The editor of a certain London newspaper is said to have threatened to sack any man on staff quoting the passage, his rant ending with "I never want to hear that joke again!". Cue everyone...
The Mikado in particular is the source of many now-familiar English phrases, such as "a short, sharp shock," "Let the punishment fit the crime," and "grand Poohbah."
Somewhat confusingly, if Gilbert wanted a word like "Navy", "Sympathy", or "Arcady" to rhyme with "bee", he always wrote it out as "Navee", "Sympathee", or "Arcadee". So, "I shall live and die" is meant to rhyme with "A heartfelt sympa-thigh", but "Stick close to your desks and never go to sea / And you all may be rulers of the Queen's Navee". It's Victorian! Iolanthe goes one step further:
Strephon: A shepherd, I -
Chorus: A shepherd, he -
Strephon: — from Arcady
Chorus: — from Arcadee
Seinfeld Is Unfunny: It is hard to believe now, but at the time what Gilbert was doing was actually quite new and innovative.
The story behind Sullivan's popular and beautiful song "The Lost Chord". Sir Arthur's brother Frederic, though trained in architecture, shared his brother's love of music and eventually made a career on the stage. He created the role of the Learned Judge in the first G&S operetta, Trial by Jury, and was quite well received by critics. However, Fred died of liver disease and tuberculosis at the age of only 39, leaving behind his pregnant wife and seven children. Sir Arthur composed the song at Fred's bedside, dating the manuscript five days before his death.
True Art Is Angsty: Guess which of the operas got the most critical praise for the composer (and was also both his and Gilbert's favourite)?
John Wellington Wells, the title character in The Sorcerer, who actually was supposed to be the villain, but unfortunately his evil is only hinted lightly upon in the text so one feels the retribution is a bit overdone. In the end there is a choice of whether he or the handsome Tenor Alexis dies; the audience tends to opt for the tenor. It doesn't help that most people can't stand Alexis anyway
Dick Deadeye, who is hated by his shipmates just because he's ugly and a hunchback— true, he does rat on the two lovers and is not a very nice guy, but many people still feel sorry for him. From Pirates on, Gilbert tended to redeem his villains.
Values Dissonance: Princess Ida is an attack on feminism that was considered outdated when it came out.
The unnamed king in the song "There Lived a King" from The Gondoliers
Arac, Scynthius, and Guron in Princess Ida, specifically during their (in)famous striptease song ("This Helmet I Suppose"), but really, any time any of them opens their mouth. ("On the whole we are/Not intelligent—/No! No! No! Not intelligent.")