Technorati tags are an interesting idea, but it falls apart under real world conditions.
Embedded Tagging Only
Because the author of a blog is the only one who can inject tags, this presents a wide open opprotunity for spammers to turn Technorati into a spam search engine - a spamgine? See Om Malik's post for more details.
The author of the post is unlikely to search and find all applicable blogs, as they just want to get the post written. Thus it's unlikely to ever become a majority action.
Tags are vague
Since any arbitrary word can get used as a tag, the probability is that the need for disambiguation will overwhelm their utility. Coupled with the lack of the AND operator at technorati, this becomes a fatal flaw in itself.
No markup tags
The author of a post is probably the last one you want to criticize the post, which is where tagging can be its most powerful. If the rest of us could tag posts, it would be far more likely to be an accurate (or somewhat fair) representation of what a post was really about. Even a popularity contest is going to be more representative than self nomination and promotion.