Plus it seems that the criteria is probably broad enough to sweep in a lot of other games. I mean, why this title and not, say, STALKER or Metro 2033? What is the magic criterion that pushes a particular game onto the index (when it doesn't have Nazi content)?

It is about violence. If the content is considered too violent (for the persons doing the ratings), you need to be an adult to buy it. And this means, an age verification would be needed, or the easy way is to just block it.

Rolling the dice. I mean Mortal Kombat X got a rating after all.
But seriously - it depends on the composition of the panel reviewing the game at the ...which only happens when didn't get a rating by the [url=https://en.wikipedia.org/wiki/Unterhaltungssoftware_Selbstkontrolle]USK.
But the important thing is that indexing a game does not result in a prohibition of sale - only in a restriction of sale (as prohibition of ads and a strict age verification - which is not a problem because age verification is mandatory when selling movies and/or games anyway)

^This, if only they would care enough. :/

BTW, despite what GOG claimed MoO is locked only in KR, not in KP (which makes sense, given that the stated reason is that they're waiting for the KR rating, which is most likely of little interest to KP).

So age verification in Germany is determined how exactly? Is it enough to enter your birthday? Or do you have to mail a copy of your id?

Wtf? When it came out, Dying Light was available in Germany, why is it locked away now?

Depends!

For games you buy digital as downloadtitles via GOG, Steam and so on
If a game is rated 18 by the USK or is unrated (which is automatically rated 18) you need to verify your age when buying it via your personal id, a payment method which proofs your age (like a credit card) or via a time restriction (yes, you heard right! A old law which was created for cinemas also counts for the internet) from 23-6.

If a game is indexed on List A commercials and selling is only allowed if minors don't have access which means you need a closed usergroup (age verification when you enter the group and when buying the game) which is basically too expensive. If a game is indexed on List B (which doesn't mean banned) it is banned.

For games you buy as retail (includes keys)
If a game is rated 18 by the USK or is unrated (which is automatically rated 18) you need to verify your age when buying it via your personal id and if you order it there is a special option (you need to verify your age when receiving it).

If a game is indexed on List A commercials and selling is only allowed if minors don't have access (not public) you need to verify your age via ID when buying or ordering. If a game is indexed on List B (which doesn't mean banned) commercials and selling is only allowed if minors don't have access (not public) you need to verify your age via ID when buying or ordering (the difference between downloadtitles and retail versions).

If a game is confiscated by a court it is handled like list B with the exception that commercials are forbidden and the selling is a bit complicated. Regarding the law the selling would also be forbidden but this was overruled several times by different courts including the highest court of Germany.

Fun fact:
The criminal codes for violence (131 StGB) and porn (184 StGB) have a special line which excludes parents so they are allowed to show such stuff to their children (doesn't matter how old).

Of course all this stuff only counts for German stores and stores outside of Germany are not effected by this.

Summed up:
If GOG would really follow the German law the German store would be nearly empty.

MaGog has just informed me that Dying Light is also unavailable in Japan.

MaGog checks all the countries of the world once a week (on Saturday) so I cannot say when this has been introduced (on addition, at the same time as Germany, before, after), but it hardly matters.

Probably related to this: http://www.thumbsticks.com/dying-light-the-following-release-dates-01022016/

Thanks both of you. Should I hold off then until it's official?

It's up to you. You can either:

1) Hold off before including it till April 21 (to confirm if it's indeed the above) or
2) Modify Dying Light's description to mention that it's regionally-locked possibly temporarily in Japan thanks to its differing release date (the article above mentions that the game will be released on 21 April on Steam and the game is currently regionally-locked on Steam, so I believe it will become available in Japan on 21 April in both stores).

I don't think so. Regional locking has become mainstream. I don't think GOG needs to confirm the obvious.

MaGog will notify me when a game becomes unlocked and I'll post here.

Sorry, looks like I misunderstood. I thought that the game hadn't hit its release date yet in Japan. If it's actually blocked, then I'll put it on the list.