Data Capture & Forms. - What should I ask for? What is the best input option to choose?

I recently had this discussion with a customer of mine & I thought it would be a useful talking point for the Eloqua community.

It's often asked of me as to what data should be requested in a form, & how much should be required at a particular stage in the funnel.

a lot of the points I raise here are pretty much common sense if you think about it, but sometimes it takes it to be said for it to be thought about.

The Simple answer is - What data do you NEED from a particular customer in a certain stage in the funnel?