That is probably one of the most wise advices I ever received.
They have a hard time seeing your vision, no matter how flushed out and clear it is. If they don't see screenshots, flash demo or a real prototype, you better have had a few successful Startups exits, otherwise... "no money for you!"
Here is my own advice, which I just gave a friend that is thinking about leaving MSFT and starting his own company: Customers have no imagination.
Maybe it is too generic, like the title of this post, but the reality is that if you explain customers your product, first, they are more likely to agree with you than not (so they don't look stupid? arrogant? negative?), second, most of the time customers don't what they want until they see it. Again: screenshots, flash demo or prototype.
If one calls himself or herself an enterprising investor, which is often the case with investors; he/ she regards themself as a creative, visual and analytical thinker- then they shouldn't have such trouble in gathering and synthesising their thoughts in an attempt to understand and visualise either a tangible product or service proposed by an entrepreneur that requests their financial backing. I think too much effort is concentrated on why an idea should fail and not be a success instead of building on it. So yeah, I think investors lack imagination totally, they're stiff or may be too conservative when they should be liberal.