If you enter youtube.com into Site Explorer, and click on the Organic Keywords report, you will find that you'll be redirected back to the Overview page:
The same goes for Amazon:
As well as other similar sites.
This is because we do not show keywords for websites with more than 100 million keywords per country due to the amount of data required.
If you would still like to perform research on Amazon, YouTube or other similar sites, try using a specific URL instead.
Here's an example (I'm using the Amazon URL of the Art of SEO book):
Same goes for YouTube (I'm using one of our SEO tutorials):