This is more obvious than it is profound, but I decided to post it merely by relevance. Several weeks ago, I debuted the Crappy Graphs user submission tool. Since then, the tool has been used to post hundreds of graphs. These user submitted graphs have since attributed to about 100,000 views on the site.
When you browse user submitted graphs, you can feel that they were drawn by different people. This factor helps lower the barrier to interaction. The effect is just like displaying 20 comments after a blog post. New visitors can see all these voices and feel more of an urge to chime in.
With Crappy Graphs, visitors feel this effect and click around. They submit. All in all, this site is seeing a significant increase in visit length and page views per visit.
Note: Today’s crappy graph was drawn using the user submission tool!