When I first click on the field to enter the tag, I see a list of some tags with their description.
When I enter some characters because I want to select a tag I know, I only see a list of possible candidates, but no description.
As you see in the second screenshot, the tags are shown without a description, but both the tags have an excerpt. The second screenshot is the one I took when editing the tags once the question has been posted.
featured has an excerpt that is the following one, but it is not visible as description when the autocomplete kicks in.
A special moderator-only tag to mark important meta discussions. A question with this tag can be seen on the main site in a special sidebar in the questions list and when viewing individual questions.
It would help if the description would be shown even if this case; users who are selecting the wrong tag would have a chance more to notice they are not using the tag they should use.
The revision number for current revision is 2011.8.13.3.