It’s somewhat possible, but has a lot of shortcomings… if a youtube video is posted as the featured link (i.e., the URL is entered as the title of the topic), which it appears it is in your screenshot, then the URL is available in the topic list and this can be used to embed the video (this will take some additional work in the theme)
The shortcoming is that if the link is simply posted in the body of the first post, it doesn’t get picked up as a featured link… and Discourse would still grab the thumbnail, but the video URL wouldn’t be available on the topic list. So it’s almost a hidden feature.
Also each video provider would need some custom logic in the theme to support embeds, so it wouldn’t work automatically for every video.
Generally this is something a custom plugin could do better. A plugin could check for video links in the original post and pull them out as the featured link automatically.
This is also fairly tricky because of similar issues, at the topic list level we have no idea if the original post has a poll… we don’t serialize any data about it. We could maybe check for poll in the excerpt, and then try to fetch the data from the topic as a separate request… but this would only work if the poll is at the top of the post and may have some performance issues.
A custom plugin would also be able to handle this better.