With the Wall Street Journal's redesign, we now have 'tag cloud' views of three different publication genres: blogs, boards and online news.
Here is a recent cloud from Technorati:
Here is a board cload from BoardTracker:
And here is the Wall Street Journal's cloud generated from search results:
It would be nice to believe that we could use these tag clouds to compare what is going on in either the respective media or the users of those media. However, the term 'tag cloud' has broken out of the constraints implied by the word 'tag'. Technorati's tag search tokenizes tags into individual 'words' (thus making it possible to search for the 'tag' the). I'm not sure if the cloud itself contains these pseudotags. The treatment of tags as plain words or strings means that the tags don't really share a common and distinct semantic space but are more like keywords. Note, however, that a search interface for tags (i.e. searching the space of tags, not the space of tagged posts) would be the proper place for this type of tag decomposition.
The BoardTraker tag cloud shows tags provided by users of the search engine. The tags are at the thread level.
The Wall Street Journal's tags are not really tags, but search terms. Here the term 'tag cloud' has come to mean the form of the visualization, not the type of data being visualized. In some sense, this demonstrates the strength of the visualization, but further weakens the notion of tags and tagging.
Christopher Brooks presented an interesting paper at the recent Symposium on Computational Approaches to Analysing Weblogs:
An Analysis of the Effectiveness of Tagging in Blogs, Christopher H. Brooks and Nancy Montanez
Which discussed some of the issues surrounding tags and the notion of folksonomies.