Vague is good for people who have played AD&D for 20 or 30 years and already have their own rules they've been using forever. For new GMs it's not at all helpful for the books to tell you to do something whithout explaining what it is.

Do 0th level clerics get 1d6 hp and magicians 1d3 hp?
Do they get those +2 bonuses to saving throws?
Do 0th level fighter get weapon mastery?
Do 0th level thieves get backstab and +1 to unarmored AC?
Do 0th level magicians get a familiar and Read Magic?

Yora wrote:

Do 0th level clerics get 1d6 hp and magicians 1d3 hp?
Do they get those +2 bonuses to saving throws?
Do 0th level fighter get weapon mastery?
Do 0th level thieves get backstab and +1 to unarmored AC?
Do 0th level magicians get a familiar and Read Magic?

Do you want zero level characters to have those abilities? If yes, then "yes." If no, then "no."

 

I think it's a valid approach to the subject.

The one thing I'd personally do different is to maybe add a short paragraph on 0th level character that spells it out like this to new GMs. Thankfully I got the pdf, but when having a hardcopy it can be really frustrating to see something mentioned, that you might assume to be explained in detail somewhere in the book, but being unable to find it.
It probably helps a lot if you are familiar with AD&D and already know that the subject of 0th level character has traditionally been left to interpreation by GMs. But otherwise it is a bit confusing.

Actually by far my favorite thing about AS&SH is how much the editing is improved over the AD&D books. The original edditing is a disastrous mess with rules being split into several fragments and hidden all over the boks, but with AS&SH I have all the rules about a specific aspect of the game in one place with barely any need to flip back and forth between inconprehensibly labled sections. This is the first time that I think I really understand how this game is supposed to work. Which may be part of the reason why the absence of explainations of 0th level NPCs suprised and confused me that much.

Yora wrote:

Do 0th level clerics get 1d6 hp and magicians 1d3 hp?
Do they get those +2 bonuses to saving throws?
Do 0th level fighter get weapon mastery?
Do 0th level thieves get backstab and +1 to unarmored AC?
Do 0th level magicians get a familiar and Read Magic?

When I run 0th level adventures, I do so without using class at all. My thinking is that characters can't do anything really cool yet, so no weapon mastery or picking pockets skill or spellcasting or anything of the sort. They're just people and can use stat checks to try things. If they succeed a lot doing something, they may want to advance to 1st level where they gain a class and whatever skills or powers go with that class.