Hi,
I've proposed a roundtable like discussion for the Hackathon on Thursday to discuss what makes high quality extensions so nice. The goal is to collect a list of things that developers like and look for when reviewing extensions, as advice for newer developers.
Even if you aren't able to attend in person, you can leave suggestions in the comments at https://phabricator.wikimedia.org/T172845.
See you then!
-- Legoktm