A counter-intuitive principle of soliciting early design feedback is that people reviewing a highly polished design may concentrate on superficial details and overlook fundamental issues.
For as long as I’ve been doing Ux work (about four years, now, since I was in college and an intern in the Microsoft MacBU) I have found Jan’s words to ring absolutely true. Getting stakeholders in your project to provide useful data to you about core interaction and visualization details in your work becomes more and more difficult as you apply more polish to your work.
If you’re interested in learning more about paper prototyping techniques, you may want to check out Carolyn Snyder’s book, cleverly titled Paper Prototyping. I haven’t read it, but it comes highly recommended by Joel Spolsky.
Additionally, I’d love to see a piece of software that could generate paper prototypes for me on the fly. I know some exist, but I’ve never seen one that integrates into Visual Studio. That would be hot.