1. Ability to shoot without taking the camera from my eye, or taking my hands off the camera. With my eye to camera, my left hand under the lens and my right on the shutter I can adjust ISO, shutter and aperture, exposure compensation, and pre view the depth of view. Plus select the AF point, lock exposure and override AF. Since my main adjustments are exposure compensation, ISO and AF point this is great.
2. Speed. Compared to earlier Canon cameras I've used, it just never has me waiting for it. When I take a string of shots on high speed (6fps) you can view the previews on the back before it has finished writing to the card. On older cameras I've used it showed me the first, then each would slowly trickle in as they wrote to the card. The fact that the camera shoots at 6fps is almost useless to me, but I love being able to set it to 3fps and still take advantage of the low blackout time and faster mirror. More time with an eye on the subject is better!
3. Viewfinder. The 95% viewfinder is nice - noticeably bigger than the smaller DSLRs. This is an area where a beginner might not care, especially if they are used to a compact camera with a horrible or non-existant optical viewfinder. For some the tradeoff of more weight and a higher price might not justify the viewfinder, but for me it is important. Despite the larger viewfinder I can see everything with my glasses on...
4. Customization. The 40D has extensive customization, as do many DSLRs. What I find most useful is the ability to store a complete camera setup in up to three immediately accessible mode dial slots. As well, a set of 6 menu items can be placed in a 'favorites' set for immediate recall. I rarely need to go into the regular menus.
I want to stress that these aren't things that Canon necessarily does better - they are just those things that I find most useful for my shooting style. Please comment on what you like most about your DSLR - how else will I ever find out what is great about Nikon, or Pentax, etc. without buying one :)
Photo credits: Brad Calkins.