Hey William,
I just bought a 40D & have used it during a couple storm chases, among other things. As for the basics, Canon improved a few features on this model that would definitely appeal to photographers and I know has helped me take better-looking pictures in general. Here are a few of my favorites:
1. 10 MP CMOS Sensor: versus the 30D's 8. Mostly just a technology improvement. Obviously not too different from the 30D, but when you're shooting clouds & lightning (low-exposure/contrast situations) & blowing up or zooming/cropping pictures, this definitely makes a difference.
2. 3" LCD: up from 2.5" on the 30D. A big improvement! You wouldn't think much about it, but the bigger screen makes shooting more comfortable all around (especially in LiveView mode)...
3. LiveView: obviously one of the most significant additions to this camera, it's the only model with as many features & improvements in its price bracket with the same viewing convenience as a basic point & shoot.
4. Self-Cleaning sensor: Another first for any digital SLR, a filter in front of the camera's sensor has an ultrasonic shake function that actually cleans itself of any fine dust particles whenever you turn the camera on/off.
If you've done your research, you probably already know about many of these features. But again they're a few that I appreciate the most, especially when I'm out chasing & hoping for higher quality pics all around, along with better camera life & performance.
Hope that helps!