* Displaying the video
* Controlling the video (although with the browser's default controls)
* Being logged in
* Listing videos
* Voting
* Commenting
* Suggested videos
* Subscribing
Actually, I can't see a single thing that can't be done without Javascript. It has no complex notification system aside from annotations (and I'm not sure anyone enjoys annotations), nothing that isn't provided by browsers. But no javascript would require page refreshes, which makes for a worse UX. Eh, we survived refreshes before. Maybe browsers could even agree on a standard to refresh only part of the pages by querying the server themselves. Throw in five lines of JS to save to localStorage the current time on the video and restore it on reload.