A lot of people say that glass matters more than the body. What makes the Canon XSi a consumer as opposed to the 40D and the 40D a semi-pro as opposed to the 1D Mark iii? The materials used to make the camera? the convenience of more buttons and ease of usability? Since consumer cameras are slowly reaching pro features, wont all SLR cameras one day be classified as standard for pro photography?
Construction and functionality.
Pro cameras will always have more functionality and be constructed in such as way as to take much more heavier levels of use.
Canon knows a 1DS/5D and maybe even the 40D will be used on an almost daily basis in various environments and designs it to handle such use.
When an individuals income is on the line, and they need THAT shot .... they will take it, even if it means shooting in the rain.
Would I take a Rebel out on the beach for two weeks shooting on a daily basis? No way, but I did take my 5D and 40D out with me the last two weeks. I took out the 40D with an " L " series lens, two of them to be exact. Why the " L " lens? Because they too are designed to be used in less than ideal conditions.
I shoot over 7,000 images in two weeks on vacation just " goofing around ".
I was shooting beach volley ball players, birds, boats, US Navy fighter jets and other planes in flight ( jets were the most challenging as they came up unexpectedly and only flew by once ), sunrises, sunsets, bright moons over the ocean. It was a great opportunity to get a lot of practice shooting in many different conditions. I had a blast, and made quite a few new friends in the process.