I started listening to podcasts in 2006, during my Ph.D., or right around the time that they started becoming really popular. There have always been radio shows, of course, but the convenience of being able to listen to shows about topics of interest on my own schedule was hard to beat. I kept trying out new shows regularly, and eventually found a good mix of goofy and serious to listen to while commuting, doing lab work, or exercising.
Unfortunately, I've found that I have to keep changing up my 'mix', sometimes because shows eventually stop (bye, bye, 1Up Network), or more frustrating, because most podcasts eventually 'go bad'. I've discussed this at some length with a buddy, and I have an opinion as to why.
I think that all good podcasts begin with a clear plan: topic(s), segment layout, approximate length in minutes, etc. After some growing pains as the participants to get used to the flow of speaking on the mic, the show gains popularity in the form of a healthy number of subscribers. Simple enough, except that it inevitably appears to go to the host's heads.
In the world of radio, people love personalities - some folks make for particularly entertaining shows and segments. However, I feel like a lot of podcasters, once they've established a fan base, seem to think that people are tuning in to listen to them and not what they're actually saying.
An example, the Tested.com podcast (This is Only a Test) is ostensibly about technology news and reviews (though they've started throwing in some very light 'science'). Their shows used to be ~1 hr and followed the format of discussing tech-related news, talking about the pros and cons of new products, then answering listener questions. As of late though, their podcasts have become more and more bloated with irrelevant junk, like lengthy discussions about the best way to cook hamburgers, boring 'inside baseball' personal anecdotes (you've moved into a new office? Good for you), and long drawn out segments about things that aren't really related to technology except in the most tortuously roundabout ways (I don't care about how nice the hotel was at Comic Con). Oh, and the shows are now > 2 hrs. I've found myself only listening to a few minutes before deleting them and it's likely that I'll unsubscribe soon.
I've singled out Tested as an example, but this phenomenon seems rather common. I'm sure that there are a decent number of people who just want to hear their favorite radio personalities talk and, to be fair, some of the best moments in podcasts come from funny non-sequiturs and tangents. Nevertheless, my criticism here is editorial rather than proscriptive: Just like word counts force you to write clearly and concisely, sticking to a podcasting plan and time-frame keeps said tangents to only the most interesting and at least somewhat relevant.
New podcasts compete for the ears of subscribers and tend to go to much greater pains to stay on track, keep quality high, and edit out uninteresting, extraneous material. Once they hit the 'big-time', at least as far as subscriber counts go, a lot of this tends to go by the wayside in what I assume is the failure to appreciate the qualities of the show that made in popular in the first place. In podcasting, like so many things, I'm beginning to think that it may be better to burn out rather than fade away.
In my experience, the most consistently high-quality podcasts are those that stick to a plan/format and edit judiciously in order to maintain high-quality. I may have a lot of opportunity to listen to shows, but there are always new podcasts competing for my time.
P.S. Another couple of podcast gripes: 1) Audio quality. The best podcasts are generally recorded in studios, though I've heard some excellent Skype-based ones as well. Regardless, even the best 'cast can be ruined by a single participant with awful call quality. If someone keeps dropping from the call, it's better to axe them altogether rather than force your listeners to put up with incomprehensible gibberish. 2) Bad hosts/guests. Not everyone is a good public speaker and there's nothing wrong with admitting that. The official podcast of a particular magazine, for example, doesn't have to have every staff member on the show out of some perverse sense of 'completeness'. Someone who stutters, interrupts the flow of the show, or doesn't have anything interesting to say drags everything down.