[@netweb Keymaster 12 years, 5 months ago]

One more thing, when making a custom importer always base it of the Example.php importer, I keep that updated with examples and any other changes I make for bbPress.

The latest (updated 3 days ago) is here:
https://bbpress.trac.wordpress.org/browser/trunk/src/includes/admin/converters/Example.php

As Robin points out, you just delete/remove the sections or fields that are not applicable to your import e.g. ‘tags’ or if your database doesn’t include the ‘counts’ for topics in forums then remove that part in the forum section.