The Leads
I've watched enough classic movies to realize that Dvorak's style of dancing in this movie was popular in the early to mid-30s. I don't get what was appealing about this sort of flat footed dancing, but we see it in movies with Ruby Keeler and Joan Crawford, and Dvorak does it as well as they do.
As for Vallee, I've never found him the least bit attractive but I guess I'd seen him primarily in his later roles. He's actually pretty good looking here and I get the appeal for viewers of the day.