Well, it's just that in most North American cities all the different definitions of "downtown" coincide in the same area. Denver seems to be the epitome of that, with all three major stadium/arenas, all major office skyscrapers, some significant museums, a state capitol building, very dense apartment buildings, the core of regional mass transit and a huge university campus with 3 different universities in it all within a couple of miles of each other. I think this partially happened on purpose and partially happened because it's easier to tear down abandoned warehouses to expand downtown than it is to build downtown-like amenities in the residential neighborhoods that fill the rest of Denver proper. Anyway, Chicago, San Francisco, Seattle, and most of the other major cities I've been to are particularly well-cored and centralized cities.
I think it has most to do with speed of growth, though. Cities that seemed to go up overnight just don't seem to have the same kind of core. The closet thing to Dubai I can think of is Las Vegas, where there is a lot of money to encourage large developments but otherwise you have a very sprawling city that grew very, very quickly; so quickly in fact that infill was implausible (the structures new projects would be replacing are themselves new), and major developments that happened close to the outskirts of town (the strip goes all the way to the airport now). Interestingly both cities are built on very barren deserts, so I wonder if the cheapness of real estate has something to do with the way they've grown. San Diego, too, grew very quickly, sprawled a lot, and ended up being less centralized.
I suppose that mega-cities are often multi-cored; New York City can be said to have downtowns in both Lower Manhattan and Midtown, and I know that Tokyo and Paris and London have old downtowns and new downtowns.
Like I always say, though, Dubai will come around over the next few decades when a more natural pace of growth takes hold. I still think the areas with skyscrapers really need mid-rises, and the neighborhoods, which seem to be economically segregated, may start to blur as new and expensive areas lose their chic to become more middle-class.
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You are right! I just answered in terms of urbanity as referring to a high density of skyscrapers. Deira and Bur Dubai are the true core, of course.
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