Ruby and Eleanor: Ruby looks like a talented amateur when compared with Eleanor. Ruby's upper body stays mostly stationary, while Eleanor's movements are more fluid. In 42nd Street, the other dancers seem to have a role in the story and their costumes echo Ruby's. In Born to Dance, the other dancers and the musicians are treated like an extension of the scenery. Their costumes blend into the set, and Eleanor is the sole focus.
Fred & Ginger:
Battle of the Sexes --- The two are on much more equal footing than is implied in their traditional ballroom sequences. Ginger can move more freely since she is wearing lower heels than usual. As others have observed, Fred is still leading her...
Versus other Depression-era musicals --- The dancing and singing occur in a more "naturalistic" setting. At least they don't appear to be performing on a stage for an audience to observe. Of course, they are most definitely performing for an audience, but in a more subtle and sophisticated way.
Possible reasons for changes in gender roles --- Women were working outside the home and showing more independence and freedom in clothes and hairstyles.
Random thought: I have always hated the saying about Ginger doing everything Fred did only backward and in heels. IMHO, Fred's dancing is so incredibly fluid, which most of his partners couldn't match. I do think Eleanor Powell is his closest equal and there is no finer pairing than the two of them dancing Begin the Beguine.
