YouTube and podcasts change my understanding of writing on the web in a few different ways. I am going to talk about each individually.
YouTube changing the meaning of writing to me because it is website that allows any person to contribute a video to it. By this I mean that the videos posted online are monitored only in a sense of appropriateness, not by content. If the video has inappropriate material, it can be removed, but it does not matter what the content or quality of the video is beyond this. I bring this up because this brings up the question that many debate, "What is art?" In the same sense, what is a good video or good video writing? Someone could throw something together quickly and post in on youtube and say, "I am a movie maker." So I would question whether or not the video can t be constituted as one by looking at the quality of the writing behind it. Also, writing for video is very different than on paper. You have to make sure that the writing fits the images presented, and that it is slow or fast enough to keep the viewer's understanding, along with their attention.
As for podcasts, I think that the writing style is very important. As I read the grammar girl entry, I found it very hard to keep track of all of the different myths she was correcting, as there were several. I found it much easier when I pulled up the script to follow along. With this being said, a writer of a blog needs to either make sure the content is easy to follow by keeping the script simplistic or very well organized, or they need to offer a script to follow along with. I then would question, is a podcast successful as a podcast if a person needs a script to follow along with? Why not just have it in written form if the script is going to be necessary? With these questions, I might re-consider what the definition of a true podcast might be.