In my experience the focus of business users and developers are usually very different which leads to communication difficulties and as you said lack of agreement on goals and priorities. It helps to employ a troubleshooter as a bridge between both camps. Someone with the background and experience in both areas who can mediate and manage both sides. There are individuals out there with this ability and they are often undervalued.

I often think of it as trying to get a treaty between the governments of two cultures. It requires someone with the understanding of and respect for both sides point of view.

I've seen this solution work out extremely well even on very large projects.

In my opinion there is a growing need to have a midfield between business and ICT.
As Mary already said, a bridge between 2 camps with people experienced in both business and ICT.
The reason is that the possibilities of ICT-functionalities and technologies have grown so quick that business people don't know what even is possible (and so they can't ask). simply said, they cann't express what they need.

At the other hand, business complexity is growing at least as fast. If, from an IT perspective,you want to use the right technology at the right place, there is a growing need to have a thorough Line of Business knowledge.

In this 'midfield' needs and possibilities should come together + near future changes in both LOB and IT have to be discussed. The outcome of this Midfield contacts and analysises should be the base of future project.

This will not only help to start more realistic project, but both business and IT have had there input before the project even starts which will help to have a better understanding.