When planning to build an enterprise data warehouse, there are many of small details ranging from hardware allocation to error reporting. So unless you plan for this correctly, you may deliver an inadequate system.
A good article to review on the issues to resolve when planning a data warehouse can be found in Craig Utley’s article within the July 2004 edition of SQL Server Magazine "Beating the Odds"
Company Politics: turf battles, lack of clear vision, bureaucracy, etc.
Lack of User Involvement: Ask the questions and figure out what is needed and what to report.
Overbuilding: Do not try to build a warehouse that has everything. A big problem with enterprise warehouses is that one tries to put all attributes into it, irrelevant of whether you need it or not.
Super cubes: related to “Overbuilding”, placing all dimensions and measures into one cube will result in poor query and processing performance.
Key Performance Indicators (KPI): related to cube design, unless you know what you are measuring – you’ve created cubes that provide numbers not metrics.
User Interface (UI): Even if you build the perfect warehouse, unless there is a good UI in front of it no one can actually access the data.
Poor Data Quality: bad data = bad reporting. What’s the point of reporting if you cannot trust the report?
Inadequate Funding: proper budgeting from the people to the resources required. Don’t forget bug fixes, training, and the fact that data warehouses are consistently evolving.