Maturity: it certainly feels rushed that in less than four years systemd went from nothing to the default init system in most Linux distributions (if I'm not mistaken). For something so basic and important you'd think a good amount of testing and bugfixing should happen before that. We are taking our time with btrfs and Wayland, so why the rush with systemd? We certainly could endure some more years with the previous init systems. And this comes from an Arch Linux user, I like my software as fresh as I can I get away with, but it still seems odd.
Compatibility: this troubles me the most, you see hard dependencies appearing like what we currently get with Linux, systemd and GNOME. When people talk about this they are usually referring to the BSD family. That's a legitimate concern, but think about this: if in the future appears a brilliant programmer who designed a free kernel which runs circles around Linux, what will we do if everything is so tightly coupled? We have to consider the possibility that we won't be running the Linux kernel forever and be prepared to switch if something better appears.