Fundamentally, the concept of a biological city comes from the ideal of a "living" city. Where the infrastructure we build is self-sustaining, to at least a degree. So that means plant life integrated with the design of the buildings and the urban layout; renewable energy in multiple forms; and a person-centric perspective on the layout of the city itself.
Discovery Channel chose Ecopolis because of the current green movement. The biggest concepts of the biological city have environmental friendliness as a result, but the vision is much larger than merely being ecologically sound. In fact, the perfect biological city would be something akin to an Arcology, just like the ones you could build in SimCity. Unfortunately, there are few who think arcologies are feasible in even the distant future. As such, engineering better cities as they are, now, is an achievable goal. It as close to an ideal city as we can manage, at least for now.
Detroit is the opposite of that ideal city. In this article from the Chicago Tribune, they put Detroit's problems into terrible focus. In it, they discuss the potential for Detroit to become America's city of the future, because everything is so bloody cheap. But being cheap isn't necessarily all that's needed. Because frequently, all that cheap attracts is the human detritus of society. Crime rates are high. Economic progress is low. Detroit itself needs a massive bailout.
But as the article points out,
"On a positive note, Detroit's homicide rate dropped 14 percent last year. That prompted mayoral candidate Stanley Christmas to tell the Detroit News recently, "I don't mean to be sarcastic, but there just isn't anyone left to kill."
At least he's honest.