There's a Debian debate page, but it's disappointing and everything systemd does is listed with equal value. Thanks to Russ Albery for making a much more balanced assessment, explaining what he likes. The short answer to the question is: SystemD makes things much easier for people writing init scripts. It wasn't about cgroups, or speed, or login managers, it was about writing easy init scripts.
Here are the major complaints he has with the traditional startup system:
* Lack of integration with kernel-level events to properly order startup.
* No mechanism for process monitoring and restarting beyond inittab.
* Heavy reliance on shell scripting rather than declarative syntax.
* A fork and exit with PID file model for daemon startup.
He furthermore points out these problems with startup scripts:
The model of fork and exit without clear synchronization points is inherently racy, the boot model encoded into sysvinit doesn't reflect a modern system boot, and maintaining large and complex init scripts as conffiles has been painful for years. Nearly every init script, including the ones in my own packages, have various edge-case bugs or problems because it's very hard to write robust service startup in shell, even with the excellent helper programs and shell libraries that Debian has available.
Those are the main things that systemd fixes for a distro builder, and probably why so many distros have switched to systemd, because it was built for them.