Harpo's solo performance of "Alone". A lovely song, and perhaps Harpo's best performance in any of the movies.
The Marx Brothers picked exactly the right opera to send up. The plot of Il Trovatore makes very little sense anyway (partly due to the librettist chopping out more than half of the story in between acts), but the music can stand up to anything.
Badass Decay: In a sense that Driftwood being fired from the Opera Company is the first time that Groucho isn't on top of things.
Crosses the Line Twice: Driftwood's rude, abusive, screwy, cheap and thieving schemes are hilariously despicable... or despicably hilarious! Choose one. Better still, take two and call me in the morning!
Designated Villain: Lasspari is at least unpleasant enough that the audience wants to see him eat humble pie, but Gottleib is never portrayed as anything but a decent man looking out for his opera and his investors. Sure, he doesn't like Driftwood, thinking him a cheat, a hustler, and a fortune hunter... but Driftwood is all of those things, and he has no particular grudge against Tomasso and Fiorello until they break into his business and start ruining his show and repeatedly injuring his head. On the other side Gottleib was absolutely ok with helping Lasspari ruin Rosa's career because she rejected his advances and only gave Ricardo a chance when he was literally his only hope.
Heartwarming Moments: True, Driftwood is a rude, screwy, cheap, swindler. But it's adorable watching how kind and sweet he is towards Rosa.
Some of the lyrics of "Bohemian Rhapsody", especially the ones relating the impending death of Freddie's character.
"Good Company," after Brian May's wife divorced him and he was nearly Driven to Suicide as a result.
Iron Woobie: The titular character in "Bohemian Rhapsody", who is apparently "a poor boy who needs no sympathy" and nobody loves. He kills a man and feels bad for it, yet at the same time "nothing really matters to [him]" and he decides to face his fate.
It's not uncommon to find people under the impression that "Bohemian Rhapsody" was originally written and recorded for Wayne's World in 1992, which happens to be a year after Freddie's death.
"Bohemian Rhapsody" is frequently thought to be the first "real" Music Video, but The Beatles, The Rolling Stones and other groups did plenty of music videos throughout the 60s and 70s. Queen themselves had a few videos before that as well. Making videos as part of their marketing strategy wasn't by any means as widespread as it'd become later (thanks, in no small part, to "Bohemian Rhapsody"), but they were certainly common enough back then to already be in common parlance (they were called "pop-promos").
One-Scene Wonder: The half-minute operatic section on "Bohemian Rhapsody" is arguably one of the first things people recall about Queen in spite of being very different to 99% of things the band did before or since.
Tough Act to Follow: Though some die-hard fans do prefer the more rocking angle of A Day at the Races and consider it an improvement, it didn't sell as well as its predecessor. Once its tour was over, the band, by sheer coincidence, decided to drop the over-layered approach and go back to basics.
What Do You Mean, It Wasn't Made on Drugs?: "39". While the lyrics sound bizarre and incomprehensible at first, reality is stranger than fiction: they describe space travel on subluminar velocities and time dilation according to Einstein's theory of relativity. Brian May, who wrote the song, is PhD in astronomy, and certainly knows his stuff. He described the song as "space folk".