Ian, I think it's disingenuous to claim that podcasting is failing. As even you said in your rebuttal comment, podcasting is following the development stages of a new form of media, then the measure of success or failure of podcasting should be based upon how successful (or unsuccessful) it has been thus far, compared to how successful previous forms of media (books, TV) were during their early development stages.

It also depends upon which metrics you use to measure success. An argument could be made for or against the notion depending on whether success is measured based on audience size or advertising revenue, or a variety of other variables. The availability of a vast selection of content, for instance, is certainly one of podcasting's distinguishing characteristics; is that a metric for measuring success?

You are spot-on with regard to the current drawbacks hindering the mass adoption of podcasting as a mainstream medium, but I wouldn't be so quick to call it a failure.