I particularly like this picture, but it's too big to fit into the post. The Galleria is often called il salotto di Milano, "Milan's living room", because it is such a favourite destination. It helps, of course, that apart from being attractive in its own right, it has destinations at each end: it connects the Piazza di Duomo, with Milan's Duomo cathedral, to La Scala, Milan's Opera and ballet theatre. Notice also that it features a lot of dining to complement the shopping.
I think that it's worth looking at the buildings that actually form the streetscape, because they are most of the reason that this works so well.
First, they are beautiful in their own right. Can we say the same thing about Rundle Mall? There are definitely many buildings that are wonderful, but there are also several that are not, and most of the street level is nothing special. Why would we want to go to the trouble of covering them? On the other hand, would adding a glassed roof and removing most of the awnings bring more attention to buildings that are worth preserving? Would we chase out tatty stores to raise the streetscape to this level - thinning out the bauble-and-trinket or mobile-phone stores? What about the largely blank walls of the Myer centre or the old David Jones?
The fact that they are all a uniform height makes it simpler to fit the ceiling. Supposing we were to cover the mall, where would we put the roof? 1 storey up? 2 storeys up? 3, 4, or 5? At some point we would have to make a choice between putting it up high enough to cover everything, and so leaving gaps, or low enough to be complete, and so abandoning the upper levels that are protruding beyond it. That's essentially what I feel happens with the awnings that we have everywhere. (My instinct would be to make the height as tall as possible, to make the space ambiguous - not clearly inside or outside - even if that leaves gaps for the weather to sneak in)
I should also say, Adelaide already has a direct inheritor of this tradition - Adelaide Arcade.
