I was recently involved in a research project looking at complexity in project/program management. I will briefly share my views on the subject here.
For me, complexity is a function of having many elements interacting in a multitude of ways with the level of complexity increasing exponentially with increases in either factor. By elements, I am referring to the full range of contributors – technology, people, information, processes and other enablers (eg finance).
Some elements that come to mind as especially important contributors to complexity in this context are: diverse stakeholder interests, ambiguity in objectives, geographically dispersed locations (incl. timezones, regulations etc), cultural/linguistic differences, novel technologies or solution architectures, the degree to which legacy systems are being modified or replaced, deployment scope (incl. volume and locations),