I grew up with the legendary conductors still alive. I remember seeing Fritz Reiner and Leonard Bernstein on TV as a child. Bruno Walter was still alive when I was 8 years old. I didn't really have a sense of who they were, but I knew that the music they were making was remarkable. And I have vivid images of both Reiner and Bernstein doing the most amazing things while conducting-- Reiner barely moving the baton, and Bernstein getting an orchestra to play by flaring his nostrils.
The "great maestro" began with Mahler, although he really just refined the image which had already been created by Wagner, Von Bulow, and perhaps a few others. But Mahler is arguably the first contemporary "maestro". He was supreme boss. His word was law.
In my youth, this model was how conductors operated. There are endless stories about conductors behaving badly, from torturing members of the orchestra through assaulting female soloists to staggering on stage so drunk they can't see.
The dynamics of the orchestra world have changed over the last several decades. Conductors still behave badly, but the days of such ruthless entitlement seem to be over. Players in orchestra will no longer tolerate abuse quietly. A conductor who staggers on stage drunk will not get hired back.
In the middle of the 20th Century, the "great maestro" was something that a certain kind of person aspired to be. I remember very well some conducting students who under no circumstances should ever have been allowed to conduct. They were not interested in music-- they were interested in being a "great maestro". This kind of person still exists, but it is more and more difficult for them to get serious podium time. Contemporary orchestras expect respect, professionalism, and artistry.
The "great maestros" were surrounded by mythology. I remember Erich Leinsdorf visiting a conducting class in Toronto, sitting across the room, and, without a score, referencing a specific horn problem on page x in measure y in a Brahms symphony. The students were in awe. But of course, Leinsdorf had probably conducted this work somewhere in the neighborhood of 100 times by this point in his life. It would have been more surprising if he couldn't be so specific.
It has also been interesting to see how the teaching of conducting has changed. "Old school" conductors continue to teach by emphasizing in depth score study. Almost nothing is said of stick technique. And while I deeply respect making the music itself the most important thing, the truth is that no one without the innate talent in the first place will evolve into anything like a conductor this way. Most conducting students need a good swift kick to the butt about their technique. Contemporary conducting teaching has recognized that knowing the score inside and out doesn't make you a conductor. If the orchestra doesn't understand what you are doing, it doesn't matter how well you know the music. Stick technique matters. The days of Fritz Reiner moving the stick a millimetre in each direction are gone.
It is a changing world for conductors. The job of Music Director has become incredibly complex, with boards, donors, governments, management, and all sorts of other non-musical matters looming large on a daily basis. It will be very interesting to see how conducting evolves as the art changes, and as financial pressures become more and more of a burden on the institution of the orchestra.