At first glance, youtube and podcasts may not seem to be related to writing at all. After thinking about it for a little while, I realized that their impact on writing, and communication in general, is quite extensive. I would argue that apps such as these are causing less of a need for written communication, on the web or otherwise. While both writing and podcasts or YouTube have their pros and cons, they work best while used together. Writing tends to be a longer process, but it allows for optimal editing and thought. Making a podcast or youtube video seems to be a quicker option, but it lacks the ability to edit every word that is said. Putting together the ease of YouTube and podcasts with a written script can be a very influential and useful tool.
So, how has all of this changed our understanding of writing on the web? It has shifted our attention from text based information to a broader understanding of writing and communication. In some ways, I think we are more likely to consider smaller sections of text as "writing" when we would have previously never categorized it as writing. In addition, we can now pair writing with visual presentation, such as here. In essence, we can present information with a powerpoint visual element without ever being there. This is particularly helpful with "how to" videos.
Overall, I think the YouTube and podcasts are a important part of communication and have minimized the need for extensive and overly descriptive writing (especially when giving instructions). There is a formality to writing though that YouTube and podcasts cannot live up to, which is why these kinds of communication can only go so far. While they are a popular way to communicate, I highly doubt they will be as integrated in our communications as email or other writing technologies.