systemd has to be pinned to -1 or your servers will get upgraded to it without any interaction from your part.
Of course it gets installed, it's the default init system in Jessie. Most users would expect to have it installed when upgrading, considering that it's a major new feature in Jessie and meant as a replacement for sysvinit.
Beware also that Apache configurations change.
Apache went from 2.2 to 2.4, which in Apache speak is pretty much a major version change. There were lots of changes in 2.4, especially making the event-based mpm the default. But there's also a lot of other changes so expect that you have to go through and possibly change a lot of configuration. I also have a number of custom Apache modules that had to be partly rewritten. But once that was done things have been running just fine.
Not to mention the Windows-like version numbering scheme!
...gnu11 instead of the older gnu89
Blame ISO. The gnu compiler modes named in the same scheme as the corresponding versions of the C programming language, C89, C99, C11.
Link to Original Source
Non system libraries are statically linked
Managing shared libraries across applications works fine in a GNU distribution where the distribution takes responsibility for all applications. With Apple's approach there's no good way to manage this, different applications might use their own specialized version of the library. At most you might have an opt-in system where developers can register the libraries they are using and the version they require, and have the system download and manage them for them.
If "you" are a one-man shop, that's fine.
If "you" are the legal department for a company with 10,000 developers, the GPL is scary. You can either blanket-ban GPL code, and make your life easy, or create a system for separately evaluating the use of each and every piece of GPL code you allow in, plus some auditing process to catch cheaters (who check in GPL code as their own work, which happens).
Cloud services companies usually go with the latter: because you don't have to share your code if you don't distribute it, the payoff is good to allow use of GPL code, and police the corner cases where you do distribute code. Blanket bans on GPL code are still common at old-school software companies.
Most non-free licenses are quite scary too, but they often get a pass since they are not that open to begin with.