I have three main issues with SystemD that might help you understand where some of us are coming from:
1. It effectively works as a monolithic replacement for several daemons, contra to core UNIX design tenets, and even though some of those sub-daemons can be swapped out with an alternative, often that works by running the second daemon in parallel - you can't actually disable the SystemD equivalent, let alone remove it altogether. This makes troubleshooting much more complicated when something goes wrong, especially if you have booted a system from a recovery disk to troubleshoot after a crash, compromise, or whatever and can no longer directly access several of the key sources of information necessary to do that.
2. Because of the growing number of packages that depend on SystemD, and vice-versa, it's creating a huge mess of package inter-dependencies that mean that it's getting almost impossible to build a stripped down and hardened server. Ballmer might have been right with his "Cancer" comment, he just wasn't specific enough: Gnome requires SystemD, $distro wants to bundle Gnome, therefore $distro adopts SystemD - and forces the default install of all the other package dependencies that go with it, thereby increasing the attack surface of the system. So much for hardening systems by removing all superflous code, huh?
3. All that cruft seems to be bogging the system down. We are currently migrating a large number (much larger than planned after initial results) of systems from RHEL to BSD - a decision taken due to general unhappiness with RHEL6, but SystemD pushed us towards BSD rather than another Linux distro - and in some cases are seeing throughput gains of greater than 10% on what should be equivalent Linux and BSD server builds. The re-learning curve wasn't as steep as we expected, general system stability seems to be better too, and BSD's security reputation goes without saying.
That said assuming that it "just works" a SystemD based desktop with everything from a desktop application down to the kernel talking through the same set of core services does sound like a nice idea. The problem is that most of us are not actually running Linux desktops; we're running servers and would just like the OS to mostly get the hell out of the way so we can get on with running whatever server daemons we are using. If SystemD were better architected - say a core PID1 init replacement, then a bunch of optional packages I don't even need to install if I want to use an alternative or not bother with at all, plus a massive clean up of the dependency hell that it has introduced - then I'd be a lot happier with it, but as it stands I just can't see including it on a hardened Internet facing server as being a remotely sane thing to do.