- The serial and parallel port backends have been removed. Need USB or LAN now to talk to your printer.
- The PHP and Perl scripting modules have been removed.
- Remote printer discovery via CUPS, LDAP, or SLP is not supported anymore (what was known so-far as "browsing"). Instead you will need to use Zeroconf/Bonjour or Avahi.
- The filter code that allowed to directly print images is gone. (Maybe just moved to a separate package.)
edit, 21/May: most of the removed code just made its way into cups-filters, as several people pointed out in comments below... so no need to panic...