The EOS 300D’s plastic body is just one of the elements used to reduce the cost of the camera, others include the use of a pentamirror in the viewfinder instead of a pentaprism, a reduction of features (although I feel that much of this is simply firmware crippling) and a shifting of manufacturing from Japan to Taiwan. Additionally Canon say that they have altered the production process of the CMOS sensor to reduce costs.
To give the camera more consumer appeal Canon has also replaced the EOS 10D’s neutral image parameters with a new default set called ‘Parameter 1’ which defines Contrast, Saturation and Sharpness as +1. Adjustment to Contrast and Saturation have the same power as the EOS 10D, however the EOS 300D’s sharpness is twice as strong as the EOS 10D. Thus the EOS 300D’s “Parameter 1” is the same as Contrast +1, Saturation +1, Sharpness +2 on the EOS 10D. The EOS 10D’s ‘Standard’ setting is called ‘Parameter 2’ on the EOS 300D.
In addition to this the EOS 300D is the first Canon digital SLR to support a new lens called the EF-S (S = short back focus), this has the same mount and electrical contacts as an EF lens but has a rear element which fits further into the camera allowing it to be closer to the image sensor. The lens elements can also be reduced in size as the imaging circle does not need to be as large, thus EF-S lenses should be smaller and lighter than their 35 mm equivalents. Note that EF-S lenses can only be used on the EOS 300D (so far) as no other EOS camera supports the EF-S mount.