Jane Powell was a really excellent dancer, as evidence by her dancing with Fred Astaire in "Royal Wedding" among other films and tv appearances after MGM, many of which are on YouTube.
I think Debbie really became a star and kind of showed the Debbie Reynolds she was going to become is in the "Good Morning" number, especially when she starts to sing the song by herself during the second interation of the melody, and then the dance afterwards. Notice how at the very end as they are tumbling on the couches and the last sting of the music ends, she actually checks her dress quickly to make sure nothing is showing -- very ladylike.
Donald O'Connor actually got the Golden Globe that year for his performance. I love "Make 'em Laugh", "Good Morning" and "Moses Supposes" if fun, even if the song is stupid, but not remarked upon too much is the opening dance between him and Gene, "Fit as a Fiddle" which is one of my favorite things in the film. You can see that as fine as dancer as Kelly is, O'Connor certainly as a tap dancer, is even better. The film kind of de-sexes Cosmo (O'Connor's character) by not giving him any sort of female counterpoint (or male, but this is the 1950s!) to pair off with at any point really during the film.
Yes, "Broadway Melody" is kind of an overlong, though well-done piece for Gene Kelly, though I do think Cyd Charisse is damn wonderful; this is what made her a star, though she had been doing small and featured parts at MGM for years. It's a shame the studio system for musical collapses just a few years later, but she's also wonderful in "The Band Wagon".
I guess Jean Hagen might have lost out on some parts, since Lina Lamont was kind of a malevolent sort of Judy Holliday role, and "Singin' in the Rain" was written by Holliday's friend Betty Comden and Adolph Green. But Holliday was already a big star with an Oscar by then to do a supporting role, and by that time she was signed by Columbia Pictures so not likely available to do an MGM movie. Of course Holliday and Hagen were both terrific in "Adam's Rib" from 1949.
I find Gene Kelly's singing ballads kind of a trial to listen to, especially in "You Were Meant for Me", but the picture was his baby, along with Stanley Donen. The title song, of course, is a classic. Story goes that he once ran into Malcolm MacDowell, who in "A Clockwork Orange" MacDowell's character rapes someone which singing "Singin' in the Rain". Apparently Kelly's and McDowell's meeting at the party didn't go well, and Kelly apparently lashed out at MacDowell, which seems unfair since it was Stanley Kubrick who directed "A Clockwork Orange" and encouraged Malcolm to sing it as his character. But Kelly was very protective of his signature song apparently.