Jeff Conklin nicely summarises wicked problems as follows:

1. You don’t understand the problem until you have developed a solution. Indeed, there is no definitive statement of “The Problem.” The problem is ill-structured, an evolving set of interlocking issues and constraints. Rittel said, “One cannot understand the problem with knowing about its context; one cannot meaningfully search for information without the orientation of a solution concept; one cannot first understand, then solve.” Moreover, what “the Problem” is depends on who you ask – different stakeholders have different views about what the problem is and what constitutes an acceptable solution.
2. Wicked problems have no stopping rule. Since there is no definitive “The Problem”, there is also no definitive “The Solution.” The problem solving process ends when you run out of resources, such as time, money, or energy, not when some optimal or “final and correct” solution emerges. Herb Simon, Nobel laureate in economics, called this “satisficing” — stopping when you have a solution that is “good enough” (Simon 1969)
3. Solutions to wicked problems are not right or wrong, simply “better,” “worse,” “good enough,” or “not good enough.” With wicked problems, the determination of solution quality is not objective and cannot be derived from following a formula. Solutions are assessed in a social context in which “many parties are equally equipped, interested, and/or entitled to judge [them],” and these judgements are likely to vary widely and depend on the stakeholders independent values and goals.
4. Every wicked problem is essentially unique and novel. There are so many factors and conditions, all embedded in a dynamic social context, that no two wicked problems are alike, and the solutions to them will always be custom designed and fitted. Rittel: “The condition in a city constructing a subway may look similar to the conditions in San Francisco, say, … but differences in commuter habits or residential patterns may far outweigh similarities in subway layout, downtown layout, and the rest.” Over time one acquires wisdom and experience about the approach to wicked problems, but one is always a beginner in the specifics of a new wicked problem.
5. Every solution to a wicked problem is a “one-shot operation,” every attempt has consequences. As Rittel says, “One cannot build a freeway to see how it works.” This is the “Catch 22” about wicked problems: you can’t learn about the problem without trying solutions, but every solution you try is expensive and has lasting unintended consequences which are likely to spawn new wicked problems.
6. Wicked problems have no given alternative solutions. There may be no solutions, or there may be a host of potential solutions that are devised, and another host that are never even thought of. Thus, it is a matter of creativity to devise potential solutions, and a matter of judgement to determine which are valid, which should be pursued and implemented.

And here is how I’ve roughly paraphrased Paul Cilliers description of complex systems:

• Complex systems have a large number of elements.
• The elements must interact.
• The interaction is fairly rich, i.e. any element in the system influences, and is influenced by quite a few other ones.
• The interactions are non-linear.
• Interactions have a short range, i.e. info is received primarily from immediate neighbours.
• There are loops in the interactions: positive and negative.
• Complex systems are usually open, i.e. they interact with their environment. Actually it is difficult to define the borders of a complex system. Therefore the scope is defined by the purpose and therefore influenced by the observer position.
• They operate far from equilibrium. Equilibrium equals death.
• They have histories. The past influences current behaviour. Must take account of time.
  Each element is ignorant of the behaviour of the system as a whole, it responds to information available locally.

It is interesting that these two perspectives don’t make much reference to each other. While there is mention of social complexity in Jeff’s work, there is little said about complex systems from a complexity science perspective. On the other side I’ve never seen wicked problems or Rittel and Webber mentioned in the complex adaptive systems literature.

Rittel, H. & Webber, M. 1973. Dilemmas in a General Theory of Planning. Policy Sciences, 4: 155-169.

Cilliers, P. 1998. Complexity & Postmodernism. London: Routledge.

Conklin, J.; Wicked Problems and Social Complexity; http://cognexus.org/wpf/wickedproblems.pdf; 25 January 2005.

(via Anecdote)