Thomas Vander Wal talks about Why Content Management Fails
Adaptive Path's Jeff Veen explains Why Content Management Fails. It comes down to a people problem in his book, which I agree with.
It also comes down to poor initial analysis, poor product choice based on the initial analysis, poor implementation, and trying to solve a people and process problem with technology, which often just compounds the problem.
This applies to any type of project. People make up projects, and if they don't work together well or if they don't come up with the right analysis to begin with, then the best project plan will get them to the right solution.
Several years ago, our company was learning about 4th Generation Management techniques that included work on group consensus, brainstorming and various modes of decision making. One of the exercises was group decision making with the story that we were stranded on the moon with a limited list of supplies (rover broke down, need to return to lander). The goal was to pick the supplies in order of importance to the mission. (Here is a description - Vignette 2)
One of the points of the exercise it to show the importance of working together and coming to common decisions. Our group never got past the first decision (oxygen!) because a forceful personality was convinced that we needed the radio -- he happened to be a former radio operator. We attempted to go forward in our decision-making, but the initial bad decision created some paralysis and annoyance for all the other decisions. In the end, our group "died" because we couldn't survive the first decision.
This gets back to Thomas' point in his comments. Poor setup for the problem (by the people) inevitably leads to poor implementation -- or to an excellent implementation that solves the wrong problem. Understand what pains you are trying to correct, and you will have a much happier project.