Too few career options below/besides professor level
For me, the biggest problem of the German academic system is the lack of career options below (or besides) a professor position. As Mathias Neis from ver.di once put it "The German academic system knows only two states: either you are professor or you want to become one." This is not only a problem for untenured researchers. It is also a problem for academia as we need these experienced people. But the WissZeitVG (including the "Novelle") stops them from working in academia after some time. Of course this rule has been invented to help researchers to get into permanent positions, but in fact it has failed to achieve this. People are kept on untenured positions until they reach the maximum that is legally admitted and then they get kicked out.
I think we need a change of mindset in academia to increase the perceived value of experience and knowledge – and not only search for the next young "high potential" and drop him or her after a few years when you realize he or she is also just a human being.