The very difficult trick is to find a virtuous feedback loop. Positive feedback loops might appear to be virtuous, but without balancing mechanisms they encourage viral dynamics (when did the virus become out role model for success?), group think and gaming.

Google's PageRank is an example of an incestuous positive feedback loop. Web pages with many inbound links get ranked higher, and pages that rank higher get more inbound links (because that's how many people find things they link to).