The main tag cloud shows most popular tags. Once the tag database gains maturity this cloud will mostly remain the same over the years. What is more interesting for users is what is happening now. Which tags are gaining most views and replies in the last X days? A tag cloud made trending tags will show users what is interesting and hot. Exposing such content is also great for SEO. @Brogan I know there is a similar suggestion, but that is a compilation of multiple suggestions and not quite the same.