The possibilities are limitless. By permitting users to register a new comment type and adding the relevant admin UI, plugins could be created to transform the comment system into a way to add reviews, contests, media sharing, etc. to WordPress sites.
This would definitely be part of the core, as the standard comment system would remain unchanged until a new comment type is registered by a developer.
By default, WordPress includes the comment, trackback and pingback types. Much like custom post types, custom comment types could be huge for turning WP into a more fully functional CMS.
Comment type API + WP = Better CMS.