Positive interoperability is adding user desired functionality - for instance, seeing location if specified by g tag in a note.

Negative interoperability is preventing a crap experience - for instance contact or mute lists being wiped.

For the latter, nostr would greatly benefit from a dedicated dev, as newbies and veterans alike are scared off from things breaking.

#nostrCI dev could develop automated testing between nostr apps that identifies behavior that is not acceptable to the end user.