Sierra had a reason to keep making "Hint Books" those days.
Earlier games are quite hard where there are some nasty puzzles that sometimes really need the hint book.
It's like making more money not just from the game, but from these hint books too.
King's Quest II had it's fair share of frustrating moments. King's Quest III is also known for it's hardness due to the time element used in it.
Even King's Quest V had it's moments. Like waiting in the room for the wizard to appear, in his castle. Who would have thought you have to do nothing and wait at a particular place to progress the story?!
Police Quest II has a moment where you have to call the operator, an actual number used in US. Coming from Asia, I had no idea what that number is. Without hint book, (or knowing what that number is) it is impossible to progress.
Quest for Glory II is particularly hard, since you have to do certain things on certain days. And it sort of loses the focus when reaching the other part of the twin city, called Raseir.