I've only listened to a few podcasts regularly, but the ones that seem to have me sticking around leave me feeling better about the interests I have. I used to listen to SMODCast but grew tired of that the more Smith sold out and lost himself in a haze of weed. In the past it was the LOST podcast, and recently it was the Breaking Bad insider one, both which gave insights into the show. There was another fan made LOST cast that was good because it had in depth discussions about the theories and whatnot. Ditto with Breaking Bad. There was a podcast I think called Breaking Good, fan made and decent enough, though I only listened to a couple episodes.
My love of Community brought me to Harmontown. I would compare Kevin Smith and Dan Harmon in that they're both jaded, middle-aged men whose mouths get them in trouble and can be very funny. Dan is more of a snob. Just loved to shit on things he doesn't deem worthy, and his tastes are odd. He is more learned than Kevin, cerebral, but stutters a lot. What they both have going for them is a blunt, unflinching honestly. What also makes the Harmontown podcast is a cast of supporting characters and some good guest stars (he's had Bobcat Goldthwait, Greg Proops, and even Robin Williams (even after voicing his disdain for Good Will Hunting)). Harmontown's best episodes get into philosophical discussions about current events and show a wide variety of points of views.
My point in all this, keep it unflinchingly honest and tell us something we don't know, something we can use to better our lives. Harmontown nowadays runs an hour and a half to two hours these days. No way you can do that now. I'd say if you can fill fifteen minutes commenting on coming and current releases and 'xax news, you'll do well.
JB's advice is sound. Harmontown does the games/trivia thing, and a few episodes in it started an ongoing D&D campaign. I'm not a D&Der myself but it was still fun to follow along with it, and is quite humorous.