- Many traditional init systems are based on a bunch of shell scripts with some metadata. Each service is started through a shell script that supports the start/stop/restart argument. While this makes the implementation very simple, it also makes it inefficient. To start a service the init system must create a new process (the shell script) which in turn invokes a bunch more processes (because almost anything you do in a shell script requires external commands) before finally invoking the service itself. This is expensive in terms of both CPU usage and disk I/O, and the latter really matters for boot time. Systemd accepts a bunch of service description files and handles the starting and stopping for you.
- Traditional init systems are about starting and stopping services. Systemd is also about keeping services running. For example if a daemon crashes, with most init systems you have to restart it manually or hook it to some external monitoring system. External monitoring systems usually rely on polling to check whether services are still alive. Systemd on the other hand just forks() the service, waitpids() on it, and gets notified by the kernel when the services crashes. No polling involved. Polling wastes CPU and reduces battery life. Polling also makes leaves a small time gap during which the service is temporarily dead but hasn't been restarted yet. This latter can be mitigated with a smaller polling interval but the smaller this interval the more CPU you use.
- Systemd supports on-demand starting of services. It starts a service as soon as a program needs it. This works by having systemd allocating server sockets and listening on them. When a client connects to a server socket, systemd will not accept() the client; instead systemd will figure out which service it belongs to, then starts that service, passing it the server socket. The service then accept()s the client. This is great for saving CPU, memory and disk I/O until it's really necessary. It saves battery life and reduces boot time. EDIT: It's not (x)inetd, see my reply.
Unfortunately most people slam GNOME and systemd for being NIH or crazy without understanding the reasons. I hope this post makes some people understand the rationale behind systemd.