Question: I've been doing some research on design choices in online communities, and I'm looking for some examples of design changes, events, feature additions (or subtractions), or policy changes that might have had an exogenous effect on user behavior on SO? 

I realize a similar question was asked about three years ago: 
[What events/policy changes etc. in SO have exogenously manipulated things like views, answers, votes etc.?](https://meta.stackexchange.com/questions/42396/what-events-policy-changes-etc-in-so-have-exogenously-manipulated-things-like-v) 

I'm looking for either updated examples, or possibly even things that were changed more than once.

Just for a bit of context, one example of the types of changes I'm looking for comes from the previously linked post:
In June of 2009, low rep users (>500) were shown a "Please consider commenting" message after down voting a post or comment. I'm interested in seeing if that message actually had an effect on user behaviors.

Similar examples would be greatly appreciated. Thanks.

Answer: 
* The [review queues](https://meta.stackexchange.com/questions/139536/new-feature-community-review-tasks-now-in-beta) have been a resounding success overall, but they have come with side effects because some users game the system to get the associated badge. See [The current review system encourages fake reviews; some people upvote everything rather than actually fixing problems](https://meta.stackexchange.com/q/149621) The problem is currently being battled, mainly with "surprise reviews" testing whether the user is paying attention.
* There used to be an "[accept rate](https://meta.stackexchange.com/questions/16721/how-does-accept-rate-work/16729#16729)" shown in users' profiles. If that rate was low, others would pester the user about it, or even go as far as downvote their questions for solely that reason. For this reason, they [stopped displaying it publicly](https://meta.stackexchange.com/questions/136951/lets-stop-displaying-a-users-accept-rate) a couple months back.
* [Has the abolition of Flag Weight affected flagging activity?](https://meta.stackexchange.com/q/120599) "Flag Weight" was a metric used to determine the overall quality of a user's flagging activity. [Discussion](https://meta.stackexchange.com/questions/84859/we-dont-need-to-see-flag-weight)

In general, Stack Overflow is an example of how to *successfully* encourage desired user behaviour, so this list is a little unfair in that it shows mostly problematic examples - I guess those stick in one's mind most because they generated the most controversy...
One thing worth to mention is the addition of [review queues](https://meta.stackexchange.com/q/139536/152859) and [review badges](https://meta.stackexchange.com/q/147912/152859). This has greatly increased the amount of people spending their time reviewing.

From personal experience, until that change the suggested edits queue was never empty, having 50-200 items waiting for review. Since the change it's very rare to see more than 10 items in there waiting for review.