You can read about it on the website of Mr. Koolhaas's firm, The Office for Metropolitan Architecture, here. The idea is to mimic the density of Manhattan on an immense scale, combining mixed uses (what else?) in the context of his celebrated concept of a "generic city," which he has defined as "the city liberated from the captivity of the center, from the straightjacket of identity…it is the city without history."

So why can't a city be a work of art? Because a work of art is the creation of a single mind that abstracts from the complexities of life to achieve a particular end, however definite or ill-defined that may be. To the extent that the artist intends to create one thing and not something else (though she may change her mind often) she shapes parts of reality to realize her vision as effectively as she can — something the artist and the engineer have in common. A living city, on the other hand, is something that emerges over time from the mostly unplanned interactions of those who live in it, creating complex inter-relating patterns — social, infra-structural, and architectural — that defy deliberate construction.