Podcast publishers can attach transcripts to their episodes. Each paragraph or word can be timestamped, automatically.
Bookmarking, highlighting and notetaking
Bookmarks in pure audio are mostly useless. When looking at your bookmarks, they are just a bunch of timestamps which give you no context about what the bookmark refers to. With an integrated transcript, bookmarking and Kindle-esque highlighting become possible. Social highlighting, like on Medium could be implemented. Copying and pasting to your note-taking app is also possible.
Read more about highlights and bookmarks.
Make it easy to tweet. Listeners can tweet quotes without having to thumb their own miniature transcript.
Read more about sharing.
To avoid information going in one ear and out the other, listeners can quickly look back at what they've learned in an episode. Or, they can look ahead to see what's coming in an episode.
Copy and paste
Listeners can copy and paste text.
Honey, I shrunk the progress bar
Player apps can underline or highlight the words being spoken as it plays. Rather than guess-clicking on a progress bar, listeners can skip around episodes by clicking on words.
Comment threads can be attached to any element or word, like Google Docs.
Read about Universal Comments, a system would allow comments be displayed within any podcast player app, not just on one proprietary social network.