The 3803 has more inputs and features than the 2803. Firstly, it has more digital inputs, and discrete power supplies for the analog and digital sections. It also has more powerful amps.
The 3803 is more technically advanced, too, and adds ALPHA 24 processing (basically an upsampler) for digital sources. One bonus is feature the 38' adds over the 28' is 0.5 dB steps for the volume. It also allows a digital delay feature to synch the sound to the picture for video (a common problem with video sources that include line doublers and other video processing is that you sometimes get delays, where the lips of actors are out of synch with the voices.).
Lastly, the 3803 also will upconvert all video sources to component video, which can be really convenient. It allows you to connect your TV or projector with just one set of cables.
You might want to check out
http://www.denon.com for more detailed info on the differences between the two models.