A few things that haven't been touched on much above.
Accounts of similar things happening or nearly happening in other European cities on NYE. (Is this bullshit or was there really some degree of organisation? Purely criminal or some political dimensions?)
Refactoring patterns of criminal behaviour observed elsewhere into something unprecedented:
swarming behaviour,
distraction tactics etc.
There are suggestions that ethnic gangs have been using similar tactics on a smaller scale in other European cities. (I've seen some of this, invasion of personal space as a distraction for a robbery attempt, near Copenhagen station a couple of years ago, so I find that plausible. Although I've seen scallies doing similar back home, so there's nothing inherently 'ethnic' about it.) The word that keeps cropping up in German accounts is 'Antänzer' which I think is referring to the tactic rather than the gangs.
There have been large scale criminal swarms (typically armed with something a bit more serious than fireworks) in other places. There were a couple of fairly spectacular cases in Brazil a decade or so ago, where the objective seems to have been to test and if possible overwhelm the abilities of law enforcement to respond.
It appears that the German cops were completely wrong-footed by the scale of the swarm, the fireworks and the nature of the assaults and hence that at least on this particular night, the state monopoly on force was shown to be clueless and helpless.
Some counter-terrorist pundits are very likely working right now on papers linking this stuff to the Paris attacks etc. Some of the Gates of Vienna types likewise.