I think it would be cool if we could see the growth of the number of question/amount of activity of various tags as a graph on the stats page.
I also would like to see a graph for the growth of questions of various tags. E.g. right now we there is only a number of "questions tagged [tag]" on the tag page for each tag. It would be nice to have a graph that visualize the number, e.g. a graph for the last six months (one point per month), updated once a week. It would look something like this: