From long and bitter experience of both infrastructure and software projects I am firmly of the opinion that all failures of IT projects come down to two things, poor planning and a lack of business involvement and commitment.
My experience shows that successful projects create a mutual dependency between IT and the Business. I have seen that this requires an unprecedented amount of team work, at least 60% of this effort involves change management and getting organisations to think about process in the business. So if you fail to get the Business and IT working together from the outset, your project will most likely experience problems, cost overruns, etc.
I am eager to gather your input. What does your experience of IT project delivery tell you about why some projects succeed and others fail so dramatically?