When you use a wildcard to search for tags, the tags which were found appear in the sidebar. For example, I searched for
https://stackoverflow.com/questions/tagged/xcode-*
(prompted by this question), which resulted in xcode-instruments, xcode-organizer, xcode-id, and xcode-archive. While clicking through these tags (and checking whether the hover tooltip would tell me the number of questions with the tag, it didn't, call that a [meta-tag:feature-request]) to check the distribution, I found a tool whcih seems to be generated by this script.
The UI
Note the vaporous and ephemeral little red circles. These appear when you mouse over the tag (bringing up the tag subscribers tooltip, which annoyingly hangs off the right side of the page) and then slide the mouse off the right side. They disappear after a few seconds; it was hard to get the above screenshot.
The functionality
The tooltip for the " button says "change into search term". Clicking it sends me to https://stackoverflow.com/search?q=, which could be accomplished instead by clicking on the page, which seems like a rather useless feature.
The tooltip for the x button says "remove from search", which would be a useful feature. Change a search for, say, javascript-* into all but one of the resulting 20 tags would be very useful. Unfortunately, it too sends me to https://stackoverflow.com/search?q=.
Summary
The UI is difficult to use. The "change into search term" feature is unnecessary; just click the tag. The "remove from search" could be useful, but currently doesn't work.
I suggest removing the 'appear on hover' functionality, the "change into search term" functionality and replacing the transient and inconsistently-styled remove button with a permanent tag/comment delete button from http://sstatic.net/stackoverflow/img/sprites.png.
