"Beautiful Girl" seems to have one purpose and one purpose alone: So Cosmo can notice Kathy among the dancers, and tell Don, re-uniting the two (well, that and make fun of the types of musical numbers and songs that appeared in musicals back then, pre-42nd Street). The costume montage is especially BLAM-ish...
Franchise Original Sin: After the proliferation of "jukebox musicals" in the 2000s, which contrived their stories around pre-existing songs, people very quickly soured on the idea. This can make it weird to realize that this film, considered one of the greatest musicals ever, was also one of them using songs from the 1920s, even if the term didn't exist at the time.
Moral Event Horizon: Lina crosses it when he threatens to sue R.F. Simpson unless he cancels Kathy's big-screen buildup and forces her to continue working uncredited as Lina's voice, thereby ruining Kathy's career, and also when it's revealed she may be trying to take over the studio.
One-Scene Wonder: Cyd Charisse in the "Broadway Melody" sequence, without even a single word of dialogue. Also Julius Tannen, playing the guy appearing in the short film demonstrating the new sound technology.
Padding: The film is only about an hour and a half long, and quite a lot of that time is taken up by musical numbers that have almost nothing to do with the plot, like "Beautiful Girl" and especially "Broadway Melody". However, those numbers are so awesome that it's difficult to care.
Special Effects Failure: When Don runs into the building that explodes, the characters off to the side change position with the cut.
Uncanny Valley: The frozen tableau of girls in the "Beautiful Girls" number is a little off-putting.
Unintentionally Sympathetic: Lina Lamont of all characters. True she was an arrogant, nasty, spoiled, petty, vindictive diva and her downfall was as much her own doing as it was the conversion from silence to sound, but she still represents the silent film stars who were put out of work by the coming of sound because their speaking voices were deemed unfit for whatever reason (being unpleasant to listen to such as Lina's or having thick foreign accents since many of them were immigrants).
Values Dissonance: Don gives a completely false account of how he got into acting, mentioning having attended prestigious acting schools while he actually honed his acting skills as a street performer and in minor, lackluster roles. This would be an utterly baffling thing to do from the perspective of a modern viewer, who would be much more likely to relate to a classic ‘underdog’ story instead of that of a person who rode his privilege all the way into glory.
Values Resonance: The reaction of the preview audience to sound being clumsily added to The Dueling Cavalier is very similar to modern reactions to films that show off the latest technological advances (e.g., CGI, 3D conversion) without backing them up with a strong script.
Vindicated by History: The film did disappointing box office and was generally snubbed by the Oscars, but is now seen as one of the great movie musicals of the 20th Century.
What an Idiot: Lina for thinking she could go on fooling audiences and derail Kathy's career by forcing Kathy to go on being her voice via dubbing. Even if the studio had been behind her in hopes of still being able to use her star power and Kathy willing to forego her own dreams by settling for the easy dubbing paycheck Lina could not have kept up the charade forever as her whole plan hinged on no one finding out that the voice they were hearing in the films was not hers. Sooner or later the truth would have gotten out and Lina's career over. Ironically Lina probably would have been better off reinventing herself as a comedienne, using her squeaky awkward voice as a source of humor. It may not have been as glamorous as the regal leading lady image she had grown accustomed too but it would have kept her in the spotlight.