I'm doing some quantitative research on SO and I'm trying to find good "natural" or quasi-experiments that have occurred or will occur in SO.
What I mean by a natural experiment is some policy change, events or even feature of the system that manipulates things like:
1) Views received
2) Answers received
3) Probability that an answer is accepted
4) Up votes / down votes
The changes should manipulate these variables in a manner that's similar to what you would find in a true experiment.
To provide an example, imagine I wanted to see if users asked more questions later if their first question received many answers. You obviously cannot just treat "answers received on first question" as an independent variable---it's almost certainly correlated with the posters characteristics, which will affect both the answers they receive and their future behavior. However, suppose by chance that a posters' question got a 10X increase in views because X happened that day, then we could exploit this shock to try to uncover the causal effect of getting more views by comparing them to others with ~X.
So, to summarize, I'm looking for some ideas on X.
Also - it would be helpful if you have an approximate date when the change was implemented or will be implemented.
Thanks for your answers - these are great. Too bad I had to pick just one answer. "Gentlemen, you've both worked very hard, and in a way, you're both winners. But in another more accurate way, Barney is the winner."