One development that's undergone much recent study in the field of engineering is the concept of design fixation, the reluctance of a designer to innovate due to their unquestioned adherence to existing ideas or due to the influence of previous designs upon them.
According to a study published by the American Society for Testing and Materials (ASTM), design fixation is often subliminal, and occurring regardless of what instructions are provided to the designer. This often results in a flawed design being, in a large part, copied, despite the designer being told specifically that it is flawed.
This concept is beginning to manifest itself in several interesting ways in regards to city planning. One of the most apparent is the idea that in order to be competitive and economically resilient, a city should strive on attracting a "creative class" of young professionals, something which recent research shows might not actually work, as opposed to trying to develop one by increasing the quality of education, public services, or low income housing.
Another way is by cities directly copying aspects of comprehensive plans from other cities without optimization or the understanding of why they were originally developed. A good example of this is the implementation of form-based zoning codes aimed at promoting pedestrian friendliness and mixed uses. This is often seen when cities do not provide a reason or direction for their codes, and implement them over large areas of land instead of applying them to specific neighborhoods.
A good example of this is "new urbanist" suburbs which are composed of rowhouses on a grid plan, but do not exist in a larger context. They must still be accessed by car, and still have minimal connectivity to adjacent roads.
One of the major flaws of the urban renewal program in the 1950s and 1960s was that it tried to apply standardized solutions to a variety of problems across a variety of cities, itself an expression of design fixation. This approach paid no attention to the peculiarities of the urban environment.
Jane Jacobs discussed several flaws with this approach, most notably, the fact that the design of these developments run counter to the way the street level is actually used by a city's inhabitants. She also noted that the interactions that take place among the residents of a city play a large part in keeping it safe, interesting, and economically competitive. Most importantly, she noted that each city has its own peculiarities which influence these interactions. The peculiarities of one city are different than those of another, and thus a planning solution to a particular problem may work in one city, but not in another.
In order to make a substantial effort at addressing the variety of urban problems that face us, such as poverty, crime, and abandonment, the reasons for them, demographic patters, investment and development rates, policing strategies, and travel patterns, must be thoroughly examined and well understood. In this way, each city is unique. A problem that is fundamentally different in two locations cannot be solved with the same solution.
This is what keeps urban design interesting, and I dare say, what makes it fun.