Extremley Multicultrual Megametropolis?

HAVE YOU EVER given thought to how an urban cityscape is usually laid out? That's ok, neither did I until about 5 minutes ago. Well, as you may be able to tell from the ensuing article, I didn't do any research on the subject, and am doing this all from my collective memory of what I know about cities, and what I've seen myself.
As I see it, city's are usually broken up into 5 basic areas: Metropolitan Areas, Low-Class Residential Areas (occasionally known as "ghettos" or "shantytowns"), Middle-Class Residential Areas, High-Class Residential Areas, and Market Areas.
These areas usually have very distinguishable differences, with Metropolitan Areas being a mix of almost every other basic area.
For each area there are different types of people, for example: people in Low-Class Residential Areas are usually predominantly African American, and live as 'gangstas' with their own sort of culture, and are usually much more violent then the people of other basic areas, forced to live with constant gang violence, drug-trafficking, and drug abuse.
People in Middle-Class Residential Areas are probably the only people with no predominant independent culture, and no actual widely-used name for their 'type' of people. Some refer to them as "normies."
People in High-Class Residential Areas have very distinct city cultures, living the luxurious, often lazy, and lavish lives of the wealthy. Some tend to be quite smug about their social positions, well others seem to think life is always as easy as it is for them, which causes them to lack empathy when it comes to other peoples problems. They tend to go by the name of "preppies."
People in Metropolitan Areas tend to be a mix of everything.

Now that you know the jist of a basic city's layout, imagine that each area was all stuffed and mixed into one big, multicultural area.
What are the advantages and disadvantages of this? What are the benefits and downfalls? What problems would be presented to the common people and the municipal government? What previously existing problems would be solved or escalated to become even worse?
A couple advantages to this sort of setup would have to be the diversity of the city and all the different types of people you would encounter, and the choices presented to you upon entering this megalopolis. A few disadvantages would be how unpredictable the people you would encounter would be, the constant cross-cultural clashes, and the easier ability to keep secret Mafia operations a long-term secret due to city officials being distracted by other problems.
A few benefits would be the economic boost given from such a productive community, the amount of work that would be available, and the educational opportunities.
A few downfalls would be the amount of work needed to keep the city running at a constant pace, the likely high price of living costs for decent homes, and the amount of pollution that would come with a city of that scale.
A few problems that would be presented to the common people would be the crowded population, the dangers of organized crime and gang violence, and the likely high cost of living.
A few problems that would be presented to the municipal government would be the risk of corruption of city officials, pollution, organized crime, gang-related violence, and keeping the city's infrastructure in good condition.
The only previously existing problem that would be solved would be the cost of getting from one area to another, well the only three problems that would only escalate to become much worse would be crime, pollution, and cross-cultural clashes.

I'll be writing a follow-up to this article, but ONLY as soon as I get 10 comments voicing different peoples opinions on the idea of creating a city that's a mix of every different basic urban area.