So by "design", you actually mean "appearance", correct?
Not to split hairs but, "design" implies engineering, ergonomics, general playability...things like that (at least to me), not just the "look" of the instrument. Since the term appears in the thread title, "appearance" might've been a better one to use, if that's what you meant.
Dang: I sound like somebody's mom: must've gotten up on the wrong side of the bed this morning...
But to answer your question: sound is, obviously, the paramount consideration. But yes, appearance also matters to me. If I'm going to have to look at the thing every time I pick it up to play it, I want to like what I see. Something I've noticed about several of the big, boutique bass makers is that, though they may be masterful wood workers, their sense of visual aesthetics leaves a lot to be desired (at least, IMHO). Just because a person knows their way around lumber doesn't mean that he has a good eye for beauty (or a good business sense, for that matter). Appearance-wise, "different" can be better, but isn't, necessarily, in and of itself. Sometimes I think they're trying too hard to make their instruments look "distinctive".
Another pet peeve of mine is finish color. Since this is totally subjective, I'm not going to name names, but there is one bass maker in particular who seems to have a really ghastly sense of color. Love the way his basses sound and his build quality is impeccable, but the colors? Almost all bilious.
And while we're on the subject of color: most basses are made of wood. I like wood and want to see the grain. As a result, I'd much rather that any color applied is translucent. If an individual bass sounds great and feels great, I'll grudgingly settle for opaque finish on the body, but if they've also hosed down the back of the neck and the entire headstock with it, I'm out. To me, it looks cheesy and suggests that they used second-rate wood. My bet is that, years ago, when they started using solid colors in the first place, it was in order to hide imperfections in the wood grain.