It's a bit late to start discussing this task, but late is still better than never.
Let's imagine Lila and the Wikimedia donors saying within a month: alright, we have invested a lot of resources on this Summit. Has it been successful? What have we achieved? Was it worth the investment of money and time? What went well, and what needs to improve?
What data should we be producing and collecting? Some ideas:
* Goals of the event satisfied (they'd need to be written down here).
* Achievements documented in sessions' tasks.
* Satisfaction survey. To be defined.
* Phabricator tasks created / active / resolved during the event, associated with #MWDS15
* mediawiki.org pages updated. Approximately; needs manual filtering.
* Media uploaded to Commons. We need a category.
* Mentions in Wikimedia channels and social media. `#MWDS15` hashtag?
* Number of participants / non-WMF / sponsored / not sponsored.
* Origins of participants / non-WMF / sponsored / not sponsored / local.
* Remote participation via IRC, mailing lists, hangouts...