1. Foreign language: I dare to say that I am pretty fluent in English, at least in the written, read and listened levels. I may have a harder time speaking, mostly because I don't do it that often. But in general, listening to an English podcast demands more of my attention that I really can give. If I'm walking on the street, this is definitely mentally tiring, and it may be harder while I am standing still in a train, as the ambient sound is too loud.
2. Pronunciation: Not all English speakers are made the same... And some accents are so thick that you can't even tell one word from another unless you raise the volume levels up the roof. For audio books this is (usually) not a problem, as speakers tend to be something like a plain English, but personal podcasts can be a mess of different world accents.
3. Time killing: Somehow. I mean, if you are presented with a written version of a 30 minutes podcast, you may read it in around 10. With speed reading techniques, in between 2 and 5 minutes, even. Of course, a podcast is supposed to be listened to while walking, isn't it? And this is time you can give away easily.
3+ Video podcasts: These, for me, are the worse idea someone had. They are OK for some tasks: presentations, classes, courses. All needing images. But what is the point for a lot other uses? In an audio podcast you have the advantage of listening to it while walking or jogging, but a video podcast is just a time sink. Not counting file size, of course.
Ok, that was all for my personal rant. Now I want you to tell me why I am wrong and podcasts are the best idea since sliced bread. Come on!