I want to notify some users (watchers, for example) about all comments, but not about all events.

If watchers are subscribers only to "Issue Commented" event, then watchers won't be notified about comment made when issue is reassigned with comment, for example.

If watchers are subscribed to all events where comments are possible, then watchers will be notified about all events even without comment, for example about all field value changes.

It would be great if users subcribed to "Issue Commented", received notifications about all events with non-empty comments.

Don't know how to easily hack that. Voted +1 for JRA-8743.