It seems like many of us here are having trouble with watching videos,
and there's also some confusion regarding the error messages. I'm
going to try and simplify things:

1. Error message: Hello, you either have JavaScript turned off or an
old version of Macromedia's Flash Player. Get the latest Flash player

We recommend: Following the steps listed at
http://www.google.com/support/youtube/bin/answer.py?hl=en&answer=57669

Additionally, you could try clearing your browser's cache and cookies
and try again.

If this doesn't fix it please continue reading.

2. Error Message: We're sorry this video is no longer available (the
steps below are also applicable to the previous error message)

We recommend: Disabling software such as Google Web Accelerator which
might be causing this issue. We have found that users with ad-blocking
software enabled may not be able to see videos due to a conflict with
the flash detection code.

Please add to this thread with quick fixes that helped you with the
issue.

3. Error message- “Your video is unavailable”-

We recommend:  Editing your video's tags, title or description. This
might help you play it. Please remember this is only for a video that
our site said was Live!. Also, it's only applicable to a video that
was uploaded by you. In this case,

For updates on this, please visit our Current Site Issues Page at
http://www.google.com/support/youtube/bin/static.py?page=known_issues.cs

Please know we're working on making all this work without a glitch and
would love to hear any workarounds from you. In the
meanwhile, we appreciate your patience, and truly regret the
inconvenience.

Thanks,
YouTube Anna