That's actually a difficult question
Most of the repeating questions come from people who don't bother reading
I must confess, that I didn't read the FAQ completely myself, so some of what I say now might already be in it
There is not much one can't find out, if pays attention to what's written on the screen (like changes in spirit caused by certain actions).
Most of what I can think of is actually something that should be put into the game rather than in a FAQ, some also in the walkthrough.
Shiva0067 is right, the main concern for a lot of new players is money.
When they have problems to get the funds for the clothes or spent eveything on cigarettes instead of getting the website running, they start to panic. Someone said he restarted the game multiple times before he gave up.
They have a problem to understand that Max is supposed to have no money and that it's ok to not get everything at once.
This aspect should be made more clear
in game.
Another point might be how to increase influence and mood with little work. Over time some new ways open, not all are aware of that. Sure, peeking on Lisa is fun, but her mood gets up if we don't even try and/or wait outside. A list of days/times when it is possible to (not) watch her change, might help.
Some don't understand how important influence and mood are in the first place. The RNG in the game can be a little bit annoying if one hates daily chores.
Also some don't get what opportunities are in general, that those are not separated story arcs/quests.
From a technical point of view
Where do savegames go
Where do the incest mod or other mods go (not that there are many, but still)
Most other current questions have to do with the current release chaos that happened when the enemy path got a update before the other paths: What's possible on war path/friendship path. For the extra version: Why some memories are locked, why it's not possible to proceed on XYZ. The hints are not always consistent. "You reached the end of the current xyz" does not always show if you are on the wrong Eric path. But there is not much you can do about that.
(And boy will some people be disappointed when after 0.9 there will be version 0.10)
I don't know how this can be integrated in a FAQ, these are just some thoughts.
ps: Don't forget the most important FAQ entry
Q: WHEN MOM SEX!
A: sod off