An incompetent PM fails to go to bat for his or her team when the devs are right and the customer is wrong. This doesn't mean that we don't bend over backward to satisfy the customer's needs, but that the PM does have a spine and is looking out for the team's interests. One example of this would be the customer demanding an unreasonable schedule that the team either can't make or can only make through unreasonable amounts of overtime. It's the PMs job to shield the team.
Along with this, a PM who doesn't understand the political landscape or is ineffective in maneuvering through it will struggle. Politics, office and otherwise, are a fact of life and a PM must be adept at building and using political capital for the good of the company and the team. A PM who says the wrong things to the wrong people at the wrong time and wastes the teams capital is incompetent.
An incompetent PM doesn't know or care about the technology or details of the project. Unless the PM is also a developer, it isn't expected that the PM have the technical background to implement the project, but he/she should have enough to understand the team's problems (and solutions) and be able to accurately represent them to others in non-technical terms.