A number of things I'd be curious to see added:

• Autocreation of table classes via Class::DBI::mysql and ilk. (Class::DBI::Loader seems to be aimed at mod_perl? If not, how do these differ?)
• Some exposition on handling joins via the has_many() call with mapping syntax, i.e. from the docs:

  Music::CD->has_many(styles => [ 'Music::StyleRef' => 'style' ]);

  is identical to
  

  Music::CD->has_many(_style_refs => 'Music::StyleRef');
      sub styles { 
          my $self = shift;
          return map $_->style, $self->_style_refs;
      }
  [download]

  (but note that your join table still requires a primary key)

• Real bridging of object heirarchies into relational databases using toys such as Class::DBI::SAK, which integrate lots of approaches such as that taken by Class::Tangram (or at least, so I hear)

Good stuff,
Matt