Good points. An additional one I've found useful is an enthusiasm meter.<p>How you define that will be application dependent but I always aim to be fairly stringent.<p>For a forum, something like, "Number of users that make at least 3 posts per week that are longer than 100 words" would be a good one.<p>Watching that number can give you an outstanding early warning system, alerting you to problems that might not be immediately obvious.<p>As soon as your most enthusiastic users stop being quite so enthusiastic you need to know.