Let me back up and punt just a wee bit here with a few things I wasn't either clear on or that i want to add to the conversation so far.
1) In the end the client is paying me money so if they want Monotype Corsiva in a rainbow pattern, I explain why that may not be the best choice, and they insist, they will get it. I've never dropped a client over what I would consider a poor design choice. I reread my post and realize it may come off as me having a bit of an ego and I really don't feel I do. I will give the client something they will be happy with...if I can make it look good based on what I have seen working in other places and feel, by experience, is the right path, I will do that. If they insist to have it their way (and some have), I will do it.
2) One thing I meant to ask is this: This woman wants this rainbow of color in a dated looking font for her header. She is aimed at women in her same demographic. So, who is to say that she is right or wrong? If it appeals to her, wouldn't it probably appeal to others in her demographic. I meant to include that for some discussion.
Now, having said that I do not agree that design does not matter to any great degree. You used Craigslist as an example. I hear that one a lot, but personally I don't think its a fair assessment. Craigslist doesn't have any reason to really have a lot of flair. They are a "gateway" for other info they aren't trying to push an agenda or make a sale. Eye candy wouldn't really have much of a place there. Also the fact they are, for the most part, a niche unto themselves, they can design how they please.
Too many studies have been done on how color, shapes, look and feel affect us. It is true that presenting information is by far, vastly far, more important than "the look"...but I guarantee that if you put up 2 sites that have identical layouts but one has a few more visual elements to it (fonts colors etc) that the styled one will get longer visits and more return visits than the other.
Either way though I do value all views here, they are great food for thought. Hopefully many more will chime in.