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:
A good model for how easy integration should be from an author's perspective would be something like Sentry (example integration docs).
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.)