Honestly, I don't know. I guess the basics are necessary: you need to be speaking clearly, without too much dead air, and you need to be interesting.
I'll add another which isn't necessary, but helps: you need to sound friendly. Listening to podcasts is a pretty intimate experience usually; while I've certainly put podcasts on speakers while painting an apartment with my girlfriend, the usual way to listen is by yourself, with your earbuds, where your only friend there is the podcaster(s).
And this is going to be more tentative, but I think one aspect of quality podcasts (or even radio shows) is that they are not one-man shows. That is, almost all of my favorite podcasts include multiple people having a conversation or asking questions: WTF with Marc Maron has fine opening rants by Maron, but we're there for the conversation with the other person.
Similarly, Planet Money and RadioLab are often framed as an explanation from one member of the team to the others there; and when you have two people already engaged in conversation, it seems more natural to bring in an expert guest into the conversation.
Are there other general issues that make a podcast good or not so good?