The CUPS 2.0 release is focused on performance and security improvements. CUPS 2.0 also ends its hold out of systemd support.
CUPS 2.0 has greater security around its scheduler, various OS X specific improvements, systemd support, support for TLS certificate validation and policy enforcement, updates to all Linux man pages, dropped OpenSSL in favor of GNU TLS, dropped support for AIX / HP-UX / OSF/1 architectures, and various other changes.
A full listing of notable changes for CUPS 2.0 can be found via the project's release notes.
There's also a brief message at CUPS.org about the 15 year anniversary of the project by Mike Sweet, the creator of CUPS who also continues to be involved still with its day-to-day development while being an employee of Apple. "That first release supported IPP/1.0, PostScript and CUPS Raster drivers, and network, parallel, serial, and SCSI printers. It even included a copy of GNU Ghostscript to support the PostScript workflow that was standard in 1999. IPP/2.1, PDF workflow, Bonjour, USB printer support, persistent job history, multiple-file jobs, authentication, encryption, policies, quotas, per-printer sharing controls, and the web interface all came later and have been refined over the last 15 years...Today our focus on printing is much different than in 1999. Wireless networking and mobile computing are everywhere. We no longer want printer drivers, but expect printers that support standard protocols and formats with fantastic output quality that we could only dream of 15 years ago. And our printing is more focused and personal."