Not all keyword assignments would constitute a hierarchy.

Yes, I actually noticed that after posting the Meditation. I ended up with some bookmarks arranged similarly to:

• Perl
  • Web
    • Programming
      • Foo
      • Bar
• Web
  • Programming
    • Baz
    • Qux

I'm not sure what you would call this. After reading the links in your post, I'd be inclined to say it's a hierarchy, but not a well-ordered one. Or maybe a hierarchy is by definition well-ordered, and this is just a crude imitation of one? In any case, it isn't ideal. Perhaps this problem could be solved by counting the frequency of groups rather than just individual tags/keywords?

By the way I belive tags are a fad. What is really needed is full text search for bookmarks (or better for whole browser history).

Perhaps for bookmarks, full text search would be a good way to find a specific bookmark a year after you added it. But that doesn't convince me that keywords or tags are a "fad" that will prove useless. They are a nice way of classifying things, and sometimes classification is actually necessary. Consider an archive of photos. You can't really search the "full body" of a photo, so some sort of classification is required.