1. What problem/opportunity are you trying to address in the marketplace?
2. What’s the high-level product concept?
3. What do you want to avoid getting into? An example: Microsoft Word is a general purpose word processing tool; it’s not trying to be a good tool for magazine layout.
4. What sorts of people do you think are going to use it?
5. What’s the planned business model? (Straight purchase vs. subscription vs. license vs. ad supported, etc.)
6. Are the people making the buying decisions your end users?
7. What are the technology options and limitations?
8. Who are the competitors? Think about everything that’s addressing the current gap, not just direct competitors. For example, Napster’s competitors include the obvious (iTunes, other digital music services), but the bigger competitors are actually physical medial sales (CDs represent about 2/3 of music purchases in the US), and stealing (~95% of music downloads are illegal).
9. What’s the definition of success for the product, and how will that be measured?

This isn’t a comprehensive list, but it’s a good start. A deep understanding of all these things is great, but you also need to be able to articulate answers to these questions clearly and succinctly (or at least have reasonable hypotheses!). I’d say there’s some room for exception, say if you have the time and resources to explore experimental concepts that have a high likelihood of failure. That’s okay, but only if you know that you’re working on an experiment that might fail.

So, what’s the tenth question I missed?