Plagger tries real hard to upgrade and normalize your feeds data. For example if you have a feed without a content (title or summary only), Filter::EntryFullText module does auto-fetch the content of the page and extracts fulltext body from there.
Sometimes we need to degrade the content, though. One example is embedded YouTube video in the feeds. When you use Publish::Gmail to read feed updates on Gmail, Thunderbird or whatever Email clients, YouTube embedded videos are not playable, due to the security reason. (Thunderbird has an option to enable scripting in the HTML emails, but then you'll have a security risk reading phishing emails.)
So tokuhirom suggested a filter to strip the embed tag and replace that with a simple A link with IMG tag to the thumbnail image on YouTube server. We named it Filter::DegradeYouTube and now I committed it to the trunk. If you provide your YouTube dev_id, which is highly recommended, it calls YouTube API to figure out the thumbnail image location.
This works really great. I'd like to create a more general, Filter::DegradeHTML framework, so other sites similar to YouTube can be degraded like this.