I was looking through the book, running a short campaign next week, and I was wondering about the NPC features. A lot, if not all, of the NPC's have features that aren't explained in the book. Should I just make up what those features are supposed to be, and on what dice rolls they would add a positive/negative die? For example, the Draugr has +Warrior of Old. No clue what that's supposed to be.

I think they left it open-ended on purpose to jolt the players and GM's imagination. Alas, this can also lead to alot of arguing. :-/ If anything else, remind the players that the GM's word is final in cases such as these. I'm going to assume "Warrior of Old" for example would add one or two positive dice to a melee attack.

It's with those assumptions that my issue is. While I like a 'rules light' system, this is more akin to having rules, but not letting anyone know what they are. Warrior of Old might also be about the armor that they're wearing, giving an extra resilience. It might also mean that they speak a different language and behave in an odd way since they're from a long time ago, adding a negative die on empathy checks. The writers obviously had something in mind, or they wouldn't have written the feature down. Why obfuscate like this?

Borrowing the zombie book from a friend, they had at least three abilities in there with different names that all did the same thing, give an extra physical resilience (two of those were actually explained, the third was a guess).

Stuff like 'Sturdy' and 'Day Job', which are easy guesses, are explained at length, but the keywords 'Warrior of Old' or 'Amorphous' etc are not. I don't mind there being very few rules. I do mind the book using vage keywords and not telling me what they are. Especially when sometimes the same end result of a feature can be the same, but the name can be different. I get that FF is basically just acting as a translator for this series, but it is definitely not up to their usual standard.