Been using my 400D for 15mths. Got a 40D 3weeks back cos I needed a backup camera. heh. Also I needed the larger grip to shoot events with flash + large lenses. I don't track fast objects so I cant comment on the AF Servo.
1. The AF on 40D is much much faster. I felt the 400D with L lenses was real fast, now its even faster.
2. Grip (not the battery grip) is more comfy. Better weight balance. I get less wrist pains now.
3. High ISO is much cleaner esp at ISO1600. With high ISO NR even less luminance.
4. 1/8000th shutter very useful when shooting in bright light with large aperture.
5. TV/AV safety shift very very useful.
Just yesterday, outdoor photography with fill flash, suddenly I have to take a shot shooting into the sun, cam auto shifts to small aperture to compensate. Of course, I have FEC at +1 as well.
6. HTP, I have it on, but dunno how it would turn out without it. Sometimes still have to reduce the highlights in PP.
7. Up to ISO400, IQ identical and clean for both cameras.
8. C1 to C3 presets useful for quick response to fast changing situations.
9. Initially 40D felt bulky, now its the reverse - the 400D feels like a toy. But its a toy that really delivers.
10. The 400D was worth every dollar I paid, got great photos from it. 40D was worth every dollar, til the 50D was announced heh.
Taking casual photos e.g. on holiday, the 400D is already sufficient, with its small comapct size. Event shooting 40D responds better, allows me to get the shot faster.