In case you've been living a cave and haven't seen the 'Awareness Test' TV ad/viral video, here it is.
http://www.youtube.com/watch?v=Ahg6qcgoay4&eurl=http://www.dothetest.co.uk/

The message is: 'it's easy to miss something you're not looking for'
Check it out.
It's a great analogy for the problem with keyword search that:
you might miss (not find) stuff that you really want to see.
Why? Because you've got NO WAY of knowing it's there.
(Unless, of course, you're psychic and so you type exactly the right words into the search box).

Or unless you have a 'table-of-contents'-type view of all the tags that describe the content.
Tag clouds give you a very limited view of the tags describing a coliection of content.
So they're great for showing what's popular, but they'll never be able to show you any hidden gems that you might be after. They're primitive. And not scaleable.

A table of contents showing hierarchical tags, however, lets you see at a glance the full richness of even a large collection of video.

See the Example in this post:
http://www.optimize-video-tool.com/2008/May/The-Vietnam-War-Big-BrotherTV-and-Context-.htm

Gotta love the Awareness Test video.
I think I've now got my 'mantra' for Tagmotion and knowledge tree tagging (with thanks to TED.com and Guy Kawasaki).
It is, of course: 'It's easy to miss what you're not looking for'.