An interesting topic. I have a different slant as I think so many were extraordinary.
What I really find facinating is that their type of intelligence doesn’t seem to exist anymore. Where is today’s Beethoven? These guys wrote celebrated compositions in DAYS from pen to performance and were unbelieveably prolific in their output (Bach, Beethoven, Telemann, Handel, Donizetti, Mozart). Today is is an endeavor that takes months or years to come out with 1 composition considered noteworthy.
Seems like we are, at best, becoming more distracted or, at worst, losing brainpower or inspiration.
My personal opinion is that people like Mozart, Beethoven, Michelangelo, Leonardo da Vinci etc. were intense and driven by the desire to do things beyond themselves. There are clearly ‘intense’ people today, but they tend to be those who are intensely ambitious and want to do things to advance themselves, not reach out beyond themselves. The kind of intensity that pushes people to look for meaning in life and existence is often denigrated in our ‘modern’ society (witness the denigration of religion and the push toward secularism). People are more likely to be passionate about who won American Idol than they are about finding the meaning in their own lives.
I've had this discussion before regarding the lack of great composers (like Bach, Beethoven, Mozart) in modern times. The answer is so obvious that many people overlook it.
The period that roughly runs from 1600 to 1850 was a unique time in human civilization. For the first time, people with artistic ability were able to completely devote themselves to their craft thanks to patronage - a practice where wealthy individuals (usually in the ruling class) would "sponsor" an artist by either taking him into their household as a servant or providing financial support.
Yet this period of time was also before television, radio, mass media, and the countless other distractions that occupy our modern lives. So these artists and composers were basically working on their craft from sunup to sundown with little or no interruption from outside sources.
It is hard to imagine today what everyday life was like back in the days of Johann Sebastian Bach or Ludwig Beethoven but if we could, we would likely find it incredibly boring. So if you were a composer during those times, well, that is pretty much what you did the entire day (when you weren't teaching students or conducting performances of your compositions).
In sort, there are simply too many distractions in our daily life to ever attain the compositional skill set of even a Handel, Haydn or Schubert. Imagine if Ludwig Beethoven was around today, fighting traffic jams on the interstate to get to and from work, checking his stocks on his home computer, watching the NFL playoffs on TV, flying to places like New York, Chicago and Los Angeles to meet with orchestras performing his works, downloading the latest John Grisham novel to his Kindle, playing Wii video games with his nephew, meeting with film directors who have contracted him to score their movies, trying to get his laptop to boot up so he can get a little composing in before the Jets-Colts game...and on and on.
While Beethoven would still be a very successful composer today based on his abilities, he'd likely never be able to produce works on the scale of the Fifth Symphony or Missa Solemnis as he did back when there were not all these distractions of modern life.
Put on your headphones and listen to Electric Ladyland or Band of Gypsys again.