It costs less and can be bought with the kit lens

Lack of Flash Exposure Compensation (FEC) on the 300d (can be set on a few flashes)

Plastic instead of metal body
It is lighter than the 10d
Less control over metering
Less control over the focus mode (one shot vs. Ai-Servo, etc.)
Lack of mirror lockup
Lack of ISO 3200

There are other differences (such as different defaults for in-camera sharpening) but I feel these are the most important.

(RP)

Here's another overview: What's the difference between the 300D and the 10D?