In any project when one is trying to build a target state solution it is important to gather the requirements in a effective way. Very often it is a challenge to gather effective requirements. With respect to building BI solutions it is always preferred to have good requirements. BI Solutions are closely tied with business performance and also provide them capability to do analytics. Here is a link, an article written by SQL Server Expert James Serra on gathering requirements for BI Solution.
It is a very exhaustive list, it provides a good perspective on how to approach requirements gathering.