Back before Web1.0 (and even now in print) reputation went with subscription. If a source was published, it had made the first credibility hurdle. The published stuff was at least worth looking at. Credibility still had to be assessed (New York Times or News of the Week?) but the context was pretty well defined.
But with Web2.0, the reader becomes the editor and needs a guide to assess credibility and a willingness to defend her choices.
There are checklists for assessing credibility out there. Find them and link to them.
But the question, "What makes for a credible source?" isnt' really nebulous and can be addressed. Here's a list to start with. We'll find that credability is a matter of context. Might be that credbility comes of where the source fits in the network of sources. Credability is contextual rather than absolute. Whether it depends on Truth or not is an open issue. It might, but even then, it still depends on outside factors, such as -
- credible friends
- linking to those friends
- even, balanced tone
- control of statement: no overstatement
- no complaints
- delivering what's being promised (think rating second-hand book sellers on Amazon.com)
- history of uses by others who are credible
- history of production