As I've mentioned before, I recently moved up to a new Canon 40D from a Canon 20D. I was looking forward to several feature upgrades and was pleasantly surprised.
I figured it would be helpful for anyone looking at upgrading to have a list of features I actually appreciate:
1. 10MP versus 8MP. This is obvious but it really does give me room to crop a bit and maintain larger file sizes for stock photos.
2. Noise. I used to have to do some noise reduction on almost any shot to clean up shadows but now I rarely have to do any noise reduction below ISO 400, even for stock submittal. This has really made my workflow faster.
3. Focusing. I really appreciate the 9 cross points - you can really use all 9 focus points even in lower light. With the 20D I could rarely make use of the additional focus points when I really needed them. I get a lot more keepers out of it on continuous focus, as well. One nice feature is that it can show the focus point used on the playback.
4. Viewfinder / screen. Just a little bit bigger in the finder, but lots bigger on the back LCD. I couldn't believe how small the 20D's was when I took it out the other day :)
5. Awake from any button on the back. Surprisingly this is great. When you pick up the camera you don't have to half press the shutter to wake it up.
6. My Menu. This is another great feature. I rarely have to go to the other menus.
7. Custom modes on the dial. This is pretty good - I have one set for manual for flash and another for continuous shooting (Focus mode, fps maxed, aperture priority, ISO 200, etc.).
8. High light priority. I haven't used it much, but worked nicely to keep snow from blowing out the other day.
9. 3fps. I like having the option of 3fps versus 6.5fps. It is very easy to accidentally fire off two shots in 6.5fps. I do like the fast speed for HDR type stuff, and action sequences of course.
10. Live view. Hey - call it a gimmick if you like but I've had about 5 times so far that I wanted to get a shot of the kids down low and loved being able to see what was going on. Can't say i'm in love with how you focus, but it is handy once in a while. Nice for macro tripod work as well (10x magnification for focusing).
11. RGB histogram. Nice - you can't really ever be sure with a luminance histogram, since it doesn't show a single channel clipping.
12. ISO visible all the time. I absolutely love having ISO in the viewfinder and on the top LCD. It is so obvious that you left it on a high ISO. i've never had shots at too high an ISO with this camera.
13. Responsiveness. It is hard to put a number on it, but the 40D is just that much quicker. If you take a sequence of shots at 6.5fps when you stop you are looking at the last one, not watching them trickle in. If you view 9 shots at a time on the back LCD you can spin the dial and it just rocks through them. I love it! With the 20D there was always an almost imperceptible lag that annoyed me.
All in all a solid upgrade from the 20D.