I remember reading Jeeves and Wooster once, there's a whole series of novels about them, and the character Jeeves is described only once in a few words in only one of the novels. Yet it's still a widely popular book. I think you're supposed to fill in the gaps yourself. Sherlock Holmes and Watson are rarely described as well. Lots of description is something that's only started to happen in modern books, I feel.
One of my favourate series, when I was a child, was the Edge Chronicles which, while it had a lot of pictures, it also never bothered to tell you the race of the main character, who was an elf like creature, until the sixth book; and even then it never bothered to explain what a fourthling was. I had to search the internet to figure that out.
I don't think it's so much about difficulty but keeping it vague so that you can have a basic picture of what the person looks like without the need of hundreds of pages of description which could take away from pace or dialogue.
If you look back at the book's descriptions you'll usually find that each character has a main point drawn up about him. It'll be either the size of his nose, the colour of his eye, the fact that he's very fat or very thin.
In 'The Lord of the Flies' that's how it happens. Piggy is fat and has glasses. The bad guy is ginger and lanky. Simon has black hair. The main character has barely developed past infancy. (AKA: he looks about 8-10)