Consumer Webwatch (a division of Consumers Union, the folks that do Consumers Reports) has published dual studies around what makes a website "credible" in the eyes of the user. And, actually, "in the eyes of the user" is a good phrase to use since the conclusions point to the fact that consumers rely more on interface and design to guage a site's credibility than on content, privacy policy or just about anything else.
The data showed that the average consumer paid far more attention to the superficial aspects of a site, such as visual cues, than to its content. For example, nearly half of all consumers (or 46.1%) in the Stanford PTL study assessed the credibility of sites based in part on the appeal of the overall visual design of a site, including layout, typography, font size and color schemes. (see the full study)
The other part of the study looked at subject matter experts (in the case of health web sites, they talked to doctors) to see how they establish credibility. It's not surprise that this audience pays more attention to content sourcing issues than interface.
41.8 percent of consumers made note of a site's visual cues when assessing health sites, compared to the 7.6 percent of surveyed health experts who mentioned such visual design issues. (see the full study)
Two very interesting studies. From my perspective at Medscape, we need to heed both at the same time. I believe, since we serve docs, that our content has to be absolutely top notch to assure credibility, but I also operate under the assumption that "docs are people too." If we start thinking of our audience as some group with no relation to the average web user, we will fail.