(start with marketing)
What are the benefits?
Who will use it? What market does it serve?
Who else serves that market?
What will the user try to do?
How do users solve this problem today?
Why should they use this software instead of the current solution?
Why should they use this software instead of the competition?
What sort of user interface is required?
Does it need a user interface?
What other projects are in progress?
When do you need it? What would happen if the schedule slipped? When will you have time to build it? who should build it?
Which features are most important? Are you sure? What would happen if you put off this feature?