One of the most annoying things about open source is you have absolutely no idea who is using your stuff for what unless it breaks.
This seems to be a big unfilled niche. There should really be a good common system for collecting and reporting aggregate statistics about features and users, that library and application authors can use to guide decisions.
I know of a number of existing open source projects that actively collect usage metrics, but these tend to be ad hoc, heavy-weight, and specialised to the project in question:
Perhaps what's needed could be described as "Sentry for feature usage metrics"?
In fact, although Sentry is primarily aimed at error aggregation, it seems very close to being a good fit for this: perhaps it can be extended to allow for good feature usage reporting and metrics, too?
(If not, it can at least serve as design inspiration.)