CSJ,
I would disagree with you in your statement that "you won't see much of a difference from a quality Canon consumer lens to an L lens unless it's wide open." Yes, you'll notice a huge difference in performance at either end of the aperture range. No question there. But, at any aperture, you'll notice a distinct difference in sharpness, contrast, and color saturation. Those things result from the much better quality glass and lens design that you mention in your following statement. When you pay for better glass and design, you get better images.
There are Canon consumer (non L) lenses that deliver L-quality images. The old Canon 100 macro, the 85/1.8, and the 50/1.8, to a lesser degree, are three that fall into that category. The rest of the consumer line, while delivering excellent images, cannot match the overall image quality of comparable L lenses.
I'm also not so sure about your suggestion that cameras other than the 1Ds are "forgiving." They all record the image they receive. Granted, the 1Ds records that image at a much higher level, but it's a relatively rare person who can or will invest $8,000+ in a camera body to get that. I have not seen either my 20D or 7D bodies be any more or less forgiving. If I send a good image to the sensor, I get a good result. If it send a poor image, I get a poor result. The camera records what the lens and photographer deliver.
Gary