Absolutely nothing! Well... as far as most "podcasts" go, they seem to be just an audio form of blogging. A lot of them are even more pointless than, for example, this very blog.
On the other hand, there is a lot of interesting audio being delivered as RSS enclosures. My personal favorite is IT Conversations. I'm listening to a number of others at the moment ... or rather I've got a lot of others queued up on the player but very few of them deserve a mention lest my one reader (hi Miles) wastes any time on them.
For the liberals among us, Air America and Democracy Now! shows are available by podcast.
But so what, right? Steve Gillmor has been beating the RSS drum for quite a long time and has now moved to beating the podcast drum. Which doesn't mean I disagree with him. One point I have heard him make happens to coincide with my use of podcasts. Drive time is where the money is in radio. Huge audiences trapped in traffic. There are only two things to do. Talk on the phone or listen to the radio. And the current state of radio in America is bleak. Two companies own some huge proportion of the airwaves. And the stuff they're pumping out is miserable. So you're stuck. Unless you have a portable music player and a convenient way to load it up with stuff that you'd like to listen to.
Podcasting/podcatching is to drive time radio what TiVo is to prime time television. It puts the listener in control. Load up the player with stuff you want and fast forward over the parts you don't care about.
That's it. In a nutshell. I really can't recall the last time I listened to the radio while commuting. No commercials, no annoying DJs, just things that interest me. An interesting phenomenon. I'm a less dangerous driver because of podcasting. My route to work isn't very congested and I used to tend to drive ... um ... a bit faster than I should. With interesting stuff on the player I drive slower so I get a few more minutes of listening time (but this is Texas, after all so I'm still going a little faster than I would if the highway patrol was in sight...).