[gregmac]

If you haven't tried this, you should. The type of feedback you get is proportional to the fidelity of your mock up.

Do a high-fidelity, fully realized html mockup, and you're more likely to get feedback about the little details like fonts, colors, icons, spacing, wording and maybe minor tweaks to the layout.

Draw a really low-fidelity mockup with a sharpie or on a whiteboard, and you're more likely to have a good discussion about fundamental layout/concepts, overall approach, etc, without anyone getting hung up bike-shedding about the details.

I think a lot of this has to do with perceived effort: if people think you spent hours building it, they're less likely to suggest a change that would throw most/all of your work out. This doesn't mean you can't or shouldn't prepare ideas/drafts beforehand if you want to, nor that you shouldn't increase the fidelity as you iterate.

I've tried many tools for this, but personally all the best and most fulfilling [early] design discussions I've ever had involved standing around a whiteboard.