You're right, of course; I am ignorant of the forms used at the time and thus imposing my modern impressions upon poor old Ottavio. From what you say, it seems that at the time opera audiences would have registered Giovanni's "lower" idiom as a sign of his debased status, maybe even his debased character as well. To modern (musically illiterate) ears, Giovanni's music is dramatic and exciting and Ottavio's makes him appear a cobwebby bore. Ohime! ;-)

As you point out below, move DG out of its period and one does encounter problems - like Giovanni's "noble" status. If one swaps "aristocratic status" with "simply rich", there's no real equivalent for Giovanni "slumming it" amongst the peasantry. The Peter Sellars updating had the Don as a Harlem hoodlum*, which threw the class tensions out of the window to detrimental effect. Even if you had Giovanni as a Patrick Bateman Yuppie Psycho type, the nearest modern equivalent to moneyed aristocracy, it still wouldn't explain how Giovanni came to be so comfortable in every social situation. Only "slumming it" works: the concept that someone's so secure in their privileged social status that it protects them from the nasty consequences of whatever they choose to do. Barring the Kennedys and the Bushes, you don't see that now.


* Yes, the "banquet" in the last act was a Big Mac and fries, accompanied by Una cosa rara, I due litiganti and Figaro on a ghetto blaster.