For my last project (B2B) I spoke with lots of potential customers on a casual basis. Once we determined there was indeed market interest, we did more in depth surveying and evaluations to determine the most important features and what pain points the customers really were looking to solve.
For my current project which is consumer focused, I have had lots of casual conversations and I record the input from them into a quick document I use to track, suggestions, concerns, and both positive and negative feedback. The hard part is that without a live product (we are in development) talking about things like: Would you pay for this? If so how much?, Do you think this would help you?, are just hypothetical and we find it hard to really judge by answers if the idea is truly validated. Hopefully when we rollout the private beta of our MVP we can do more in depth usage analysis and collect more accurate feedback.
The best idea is one you can sell to customers today then generalise later to a scalable business model.
I suppose I could've done some more research before I built the social network first, but my first month of sales have been stronger than I expected: http://blog.leftnode.com/entry/first-month-kwolla-sales-repo...
On the other hand, as much as I dislike Steve Jobs from his personal reputation, if he said something was a great idea that would hold a lot of weight with me - enough that I'd probably try to build a product on his recommendation alone.