I listened to podcasts a lot when I first stumbled across them. I wonder whether, given the ease of posting video online whether these will decline in popularity over time?
For me, the decline has simply been accessibility. I used to listen to a number of hobby podcasts (RPGs and boardgamers) and still subscribe. The reason that I rarely listen is that they are long (typically 60 – 90 minutes) and my mp3 player (being of the cheap variety) won’t bookmark the spot that I was up to. Any interruption and I need to start again.
I think that suggests to me that, ideally a good podcast should in general be relatively short, pithey pieces. Radio works like this. Apart from special performances or the odd long interview a piece is typically 10-15 minutes at the outside. Too long and people will start to tune out.