Take, for example, the traditional baby orc dilemma. With some groups it can inspire great play (arguments and other interaction among characters, usually). Is it a challenge? One can see it as a challenge to party integrity (if there is a party), or a challenge targeted at the moral nature of the characters. There is another way of seeing it, though: Given this interesting situation, what kind of people are the characters? Cold-blooded killers, perfect saints, something in between?
The same question can be applied to any situation involving surrendering or captives of war.

Another, different, situation: Put the player characters in a position where they have several right options (or essentially unlimited options) and see what they do. For example: PC is a hatchling dragon. Where to build a lair and what kind of lair? Again, this can be seen as a challenge, yet need not be seen as such. One game I might be running in the future: You are the only vampire(s). You are hunted. What do you do?

To take less obvious example: Very powerful wizard is approached by guardsmen out to arrest the wizard. It is clear that the guards are no challenge, but the situation is still interesting; the wizard may slay them all, negotiate, disappear, do something else, and every action has interesting repercussions.

This all is not to say that challenges are not usually interesting; they are, or at least challenging situations are. My point is that there are other ways of seeing play and sometimes those other ways are more useful.

In my definition of Hard skills, I was really focusing on the game mechanics of more traditional RPG’s. And I would not consider a skill being both hard and soft, so you are right again.

As for delivering challenges to the players, I think that the definition of challenge may be important. There are a lot of ways to challenge a player. In the most mechanical sense, in a game like D&D, a challenge can translate to a monster-based encounter, where your hard skills are put into use to create a mechanically interesting opponent, and to utilize the rules that apply to this creature (spells, abilities, etc.) to their best form.

But challenges show up in other forms, be it a mystery, a puzzle to solve, an NPC to convince to sway to your cause. I think that challenges are a core component to most RPG’s. In one way our another, there is typically an opposition mechanic at play, be it overt or covert.

I would think that a GM would want to create challenges that were enjoyable to the player. A challenge, that the player knows that he can or cannot win, at the onset , is not entertaining. The best challenges are the ones that make a player really work for the solution. Be it the right spell to defeat the monster, or just the right thing to say to the Duke to have him commit his troops to your cause.

I think a GM that does not want to create meaningful and entertaining challenges, regardless of game, is not playing as entertaining of a game as they could.

You also assume that a GM would want to delivar challenges to the players. I don’t think that this assumption is always true.

Granted that mastering the Hard Skill of Rules Mastery is easier with a rules light game, but to be effective with the game, you must really understand the rules.

My best example of a rules light game I ran, was Amber:Diceless. The game itself is not mechanically difficult, but to really deliver challenges to the player you have to understand the what the system is designed to handle and what it does not handle.

For example, Amber does not have a skill system. It assumes that all the Amberites have had ample time to acquire any necessary skills that they may need. Knowing this, you have to decide how mechanically you want to handle something like a car chase, when there is no skill for driving a car. In addition, there is no Dexterity-like skill in Amber. But by understanding the few rules of the game, you make the car chase a Warfare challenge, and resolve the car chase, in the same way you would a sword fight.

So I would not say that Rules Mastery is optional if you are playing a rules-light game. I would say that it is easier to archive Rules Mastery than if you played something more mechanically complex, like Iron Heroes.