I know I'm stating the obvious, but - use a checklist for requested functionality, with a radio-button 'importance, 0-5' modifier...

Oh - make sure to include a text-box where those polled can add suggestions of thier own. A nice twist might be to add those suggestions automatically to a 'suggested functionality' section of the poll, put that up with the same checkbox and 'importance' modifiers...

Just an idea...