One of the largest barriers, if not the largest barrier, to playing an RPG is learning the mechanics. Human beings are so unpredictable in the way they use words, understand words, and act upon words. For instance, back in the 90's I played ADnD with three different groups. Each group interpreted the text differently. Everything from the number of spells wizards got per level to how to roll initiative was slightly different- not because of house rules, but because people just read the official rules differently. IMO, this sort of thing is not all that desirable. So how can we avoid it?
First, examples play a key role in helping people understand new content. Giving players a model to follow offers them a template for their own experiences with your game. There are three types of examples I'd like to talk about for a moment: Generic, Faux-play, and Actual Play.
Generic examples are quick bites of information just to make an abstract idea concrete. For instance, if you were to give an example for Stats, you might say "John's character has a Strength of 5, an Agility of 4, a Logic of 10, and a Charisma of 8." That's a Generic example. Usually, you are not recounting anything that actually happened while someone was playing the game. It doesn't matter what the numbers were or who the players were, and the example will have no further bearing on the text beyond the section where it is included.
Faux-play examples are very common in RPGs. They often appear at the beginning (like my own Ember Twilight), during the resolution chapter, or at the ending of a book. They are usually written as a script where players are sitting around a table talking through their play. Sometimes, they include an over use of words like "Awesome" or have lots of exclamation points. Such an example might go something like this:
GM (rolls and 18): Okay, the orc swings and hits you with his club. 5 Damage. Your turn, Jake.
Jake: Ouch that hurts. I'm going to swing back. (rolls a 17). I hit!
GM: Nice hit, Jake! He takes 8 damage and dies.
I call this a Faux-play because it's totally manufactured. This is hardly how people play an RPG, but it is how play is commonly portrayed in an RPG text. However, these kinds of examples are still useful. They can demonstrate the order of mechanics and how various parts of the system work in concert with each other.
An Actual Play example is when a writer uses an Actual Play report, either one posted on the Internet or one he wrote for his own reflection, and includes that in the text. This isn't done too often. However, an Actual Play example provides the reader with an authentic model for play. By reading an Actual Play report within the text of a game, the reader will gain much more insight as to how the game truly works with real people in the real world. This can be invaluable especially if the game has a lot of complex mechanics.
There is room for all three kinds of examples in an RPG. A writer should be aware of each and use them appropriately.
A second way an RPG can teach its mechanics is to be aware of how learning takes place. Over on Story Games, Doyce Testerman wrote a series of posts talking about adult learning styles. Rather than replicate his work, I'll link them here for you:
[Teaching Your Game] Rules for Teaching Adults
[Teaching Your Game] Teaching Specific Tasks
[Teaching Your Game] Attention Span and Effective Use of Time
[Teaching Your Game] Readability – The Fog Index
[Teaching Your Game] Using Adult Learning Techniques to Deliver Game Content
Finally, a game can teach its mechanics through a tutorial. Some games turn themselves into graphic novels that visually and literarily teach the mechanics to the readers. Sometimes a game will offer a step by step procedure for the players to follow to create and use their characters for the first time. This is most common on CRPGs or MMORPGs, but tabletop games can do the same. It becomes a matter of writer's choice.
It is very important to be aware that a game must do its best to teach the mechanics it contains to the reader. Failure to provide clear examples or failure to understand how people read and learn will cause real problems when people sit down to play your game. If you would like to test the "learnability" of your game, give it to someone who's never seen it before and ask them to make a character and tell you how they will use that character in play. It might just surprise you.