Most "other" storage is app private storage. If an app has some reason to create a file on the phone (that isn't media), it'll be cataloged as "other".
Any book you download in the Audible app falls under this category, since Audible books aren't available as regular audio files.
Another big offender is the Navigon/Garmin app. It doesn't use the same maps as the built-in or Nokia map apps, so any Garmin offline maps you download will similarly be app-local, and therefore "other."
The main change in GDR3 isn't so much the elimination of app local storage, but the Phone Storage settings applet now includes said storage in the "apps and data" category, rather than separating it out into the FUD inducing "other" bucket.
As always, in the Lumia Storage Check beta, you can get some sense of which apps are hogging the most storage by clicking on the apps summary to see the list of each app and its data sorted in descending size order. Obviously, Audible and Navigon lead the list on my phone.