Browser - Chrome (latest version - 10.0.648.204)
On http://www.youtube.com, the player never displays - the whole Flash area (including where controls should be at the bottom) is just black. Strangely, the exact same videos embedded into other sites work fine, as they do when changing the URL to https://www.youtube.com.
I've tried clearing all YouTube cookies; and even clearing Windows 7 Basic internet cache, uninstalling Flash entirely (in case it's interfering with the internal implementation in Chrome), disabling Flash hardware acceleration.
As I have a workaround it's not essential, but it'd be an interesting one to pinpoint - any ideas? (Would obviously prefer not to delete all my cookies, sign-in data etc for no reason!).
Edit (10th Apr 2011) - ok, so it's now working again, somehow(!). Apologies that I wasn't able to pinpoint what changed, but I'll keep an eye on this post, and definitely let everyone know if it happens again...