I think, there is one very simple and organic way to solve the problem of content discovery and topics based feed, at least to some extent.

Just let users add tags to each note while posting, show auto-tag suggestions, and encourage them to do it. 

Then implement a web of trust to eliminate spam, and if the WOT is based on users' followings, that feed will become even more relevant.

Sooner or later, we have to categorize the content so people can start following topics and build communities around them.

It will be community-driven categorization of content, with no black box algorithms involved, and not that complicated to implement fir clients; 

Snort has already done some work in this direction & it also seems like a right direction.