Tracker bug for issues that prevent playing YouTube videos. My test case for this is a fresh install of Fedora Workstation. The main difference to Debian (where I'm told YouTube works fine) is the absence of the codecs that Debian distributes in its gstreamer1.0-libav package, which we don't have because unfortunately it is illegal to distribute them in the US. So if you can't reproduce these issues on Debian, try uninstalling that package first.
Can we close this now?
Some very old (many years old) and very new (1-2 days old) YouTube videos are still broken, but this is Google's fault for not offering WebM for these videos; nothing we can do about that.