Stephen Turbek writes on the need to make wireframes more "real" in the B&A article: Real Wireframes Get Real Results.
Real people don’t understand wireframes
Usability tests are done to get early feedback on content and functionality decisions from people outside the project team. These participants, unfortunately, are not sure how to respond to a wireframe. It is not intuitively clear what they should be doing, which site they are looking at (public site, intranet, client site)—it may not even be clear that they are looking at a web page. This lack of information and
context adds a bit of cognitive friction to each step in the process. This level of confusion results in less confident answers and fewer opinions.
The comments that follow the article are more useful in that it provides a good set of pros and cons for testing with low-fi and high-fi wireframes and prototypes. For example, with a high-fi prototype testers may expect the functionality of the prototype to work but on the other hand users don't have as much context available to them when testing with low-fi wireframes. All seem to agree that you need to manage the test participant's expectations and provide context. Use of real text and making links and buttons looks clickable is a must for all types of wireframes and prototypes.