One factor is that you want people to be able to use their existing film lenses on a DSLR, that will influence the design. There is an ISO adjustment on a DSLR, I keep it set relatively low to get reasonable shutter speeds and apertures for depth of field control. Of course, ISO is really just a film metaphor. The shape of film bodies is pretty familiar and comfortable for most people; there were some flaky shapes at first, but they weren't very popular.