After tagging all my papers, I want to be able to see a bird's eye view of all my tags at once. Which tag has the most papers? Which tags overlap with each other? Does one of my tags turn out to be an subset of another? After seeing this view, maybe I'll decide I want to merge some of my tags. etc.
Such a view could look like a network graph where each tag is a circle (node), and connecting lines between circles have a weight or thickness to show the number of papers with both tags. Each circle size is determined by the number of papers with that tag.
If the Papers app doesn't have this visualization, it would be good to at least have an export of (tag, paperID) pairs so that I can create my own visualization in another tool.