What's true for doctors applies equally to consumer electronics manufacturers: first, do no harm. Canon is usually good at adhering to that maxim, making only minor changes to successful products and saving the daring moves for the models that need it.
Changing sensors isn't normally considered terribly daring when it comes to digital cameras. But as the EOS 400D's predecessor -- the EOS 350D -- is renowned for producing excellent, low-noise photos at a more-than-adequate 8-megapixel resolution, it seems risky to replace it with a higher-resolution but potentially lower-sensitivity chip as Canon has done with the new model. Perhaps the Nikon D80 raised the stakes. Perhaps Canon felt it was an inevitable necessity. Whatever the reason, it yields mixed results.
Sticking with similar sensor dimensions allowed Canon to keep the same moderately compact design for the EOS 400D, though it weighs 25g more than its 485g predecessor. With the small, exceptionally light kit lens, the camera felt well balanced. Attached to the substantially larger and heavier 16mm-to-35mm (25.6mm-to-56mm equivalent) lens or the Speedlite 580EX flash, however, the 400D does feel slightly lopsided.
Although much of the design remains the same as the 350D's -- it comes in either black or metallic-silver plastic -- there are a couple of key changes. The LCD display has grown from 46mm (1.8 inches) to 64mm (2.5 inches), essentially squeezing the status/info LCD into the ether. On the one hand, using the main LCD allows for an exceptionally readable, in-your-face method of monitoring the settings. The paper-white background gets distracting, however, and the automatic sensor -- which blanks it when you put your eye to the viewfinder -- makes it even more so. You can turn it off altogether, but the info in the viewfinder doesn't include ISO speed, white balance, battery level and other useful settings that generally display on a status LCD.
In most other respects, the control layout on the 400D mimics that of the 350D, which is pretty much how it's been on Canon dSLRs since the beginning. That's a consistency we can appreciate. The 400D can also accept all the same accessories as the 350D.