Till there was rock, you only had God
- David Bowie, "Sweet Head" (1972)
It's also the case that a step-change here, in that while fame was the object of Hunky Dory, here it's very much the subject. Bowie is no longer content to look at fame from the outside, but to examine it at first hand. The major hurdle that had to be overcome was that David Bowie was not famous, and the key realisation was that fame could be constructed and worn, like a mask. One of the things that's so interesting about the whole Ziggy Stardust phenomenon is the role that performativity plays in it: Ziggy Stardust is a rock star because he tells you that he is, and because he does the things that you have come to expect from the cultural script of rock stardom (In 1972 the script was only about 15 years old, which makes it interesting that the Ziggy Stardust character was able to capture the concept so perfectly and prefigure so much of what was to come: in 1972 Elvis had Five Years left).
In some senses what seems most remarkable now is Bowie's decision to kill off the whole Ziggy Stardust mythology after only a year, at the height of his (now real) stardom. In some alternative universe there's a David Bowie who milked it for all he was worth, touring the world with The Spiders from Mars for 5-6 years, and who in 2012 is a mostly-forgotten piece of nostagic kitsch, like Alvin Stardust. The decision to kill off Ziggy was gutsy, and it resulted in Bowie being able to repeatedly change himself, and take an audience with him. Rock stars and identities are ephemeral, and if you've completely created yourself once, you can do it again and again.