I agree with Emre - form follows function. That said, asthetics is a tough thing. There are makers whose work just pops out at you - Raymond Richard for example - as just plain beautiful. Other knives look stiff, even if they're obviously functional. I've never been able to quantify what makes this true. There are things like the handle being the same width as the blade at the guard juncture, and general handle to blade length ideals (i.e. the Golden Ratio) that can be applied, but generally this is a very subjective issue. Some people have it - and some don't. I think most people can judge for themselves whether a given knife "looks right" and those who can pull that off more often than not will continue to make knives. Those who can't will eventually move on to what it is that they're best at.
My personal taste runs to simple, organic shapes. There are knives with all kinds of tricky grinds and cutouts in the blade etc, but put a simple straightforward bowie next to a fancy one gets my attention every time. But there are as many personal asthetics as there are people, and because of this there is a ready market for all kinds of knives. What's most important for a knife maker is to develop a personal style that is recognizable, and I think that's inevitable if one makes enough of anything. The tools available and the way the craftsman uses them do a lot to create a given maker's "look."