One of the designers at Technorati talks about his perspective on tags. A very interesting read.

Tags are the first major interface to our living database that's truly browsable. Just click a word and see what's there. It's fun. It's rewarding. Even typing a word into the search box to try a new tag is enjoyable. The experience is pleasant enough to reward risk-taking. There's always something fun to see because even when there are no results, there's an invitation to participate.

They're bottom-up, so the classification comes from the people who make the content, not some highfalutin academic. They're flat, not hierarchical, so they avoid the pitfalls of hierarchical organization. And they're emergent - a thousand monkeys with a thousand typewriters and all that.

But other people have already talked about all that, but what I find truly exciting about tags is that they're all about browsing. And not the directory/library/annoyingly hidden kind of browsing that led to the death of the Yahoo Directory and the emergence of the single Google box - the fun kind of browsing, like shoe shopping on Haight Street.

The results are formatted differently. Instead of a robotic list of results, the tag pages are alive with content. Just compare the Apple tag page and the Apple keyword search to see what I mean. Of course this is due in large part to the wonderful photos from Flickr. But there was also a conscious design decision on my part to make the pages feel like content, not search results (even though they are auto-generated). I wanted each page to read like the tag's hometown newspaper. "Here's what's new in Travel section of the real-time web today."