Increasingly projects are starting where the end outcome is not so clear. Instead of a project being started to deliver a CRM system, the project today will be about improving average customer spend. Instead of a project to build a dashboard report the project will include identifying what should be reported on and so on.
When you start a project and the problem is still only partly defined, you typically can't have a clearly defined solution. That's the rationale behind the Agile Manifesto which suggests you build iteratively and work out both the problem and solution in increments. It also applies to decision-point-gated, waterfall type projects.
What it means for the project manager is that instead of focusing in on the work and spending your energy on making sure the job is well done, you spend your time managing external expectations. Rather than working the plan you are mediating the expectations of both the project team and the client and stakeholders. Both sides of the see saw will need to have their expectations moulded so that over time they converge.
When everyone is working to the same vision you have a chance at success. To get there you need to do things to make the software or business process tangible. You can use demos, prototypes, and pilots as tools for this.
Feedback from presenting these incremental developments becomes vital for the project team. Where possible have them in the room with the stakeholders when you demo your new tools. Hearing the feedback directly helps them get a sense of what is important and how the development needs to be adjusted.
Another thing you'll need to consider is that you'll be so busy managing stakeholder expectations and relationships that you won't have so much time to manage the team. In project like this having the right people is even more important. They need to be able to work without your leadership and monitoring them on their work. They also need to be pro-active and knowledgeable about project management processes so that things like requirements, risks and changes can be properly managed. Experience will count here.
As I intimated above, it's a real challenge to get to the end of this sort of project because the end state wasn't clearly understood early on. Basically you are not going to be done building until the client has something they are happy with (and can achieve their business case with.) You, the PM, need to manage them to that happy place and it's not an easy task.