The stepping stone idea isn't a bad one, but that's the interesting thing about games: Stepping stones are "supposed" to be implemented in so many titles already.
While not in every game, there's been differing difficulties, in an attempt to make the game both more approachable and longer lasting. Personally, I'm a proponent of having all difficulties unlocked from the initial outing, for one matter. Silly things like unlocking an easier mode after dying several times, that's just a bad decision.
Now, then, the reason I suggest using difficulties, rather than simplified games makes a bit of sense, when you think of it. SilverGrey mentions the library, which solidifies this: When someone picks up a book they're learning with, they plan to take it back about as soon as their done with it. The childrens' books parents and kids buy are bought for reasons not based on difficulty, but more on cuteness, intrinsic value of the plot, familiarity of the characters, or any other number of reasons. Yes, the books are great for beginning readers, but they're books that will be read again, even though there's no educational value. What I'm shooting to say is that developers need to refocus on creating experiences that gamers of all types come back to play again.
So really, I'm not sure if an icon or metric of game difficulty is the right way to go. There's a stigma, at least in my perception, that books with a similar system are often just to learn with, and of no other value. I don't think that's the direction you want to take with expensive titles, though it could work with low-budget titles, still.
With that said, I'll say what I believe to be the case for the gaming industry, in particular for new gamers: Shorter games, focused on gameplay over story, with lower budgets and lower prices. I'm not saying that longer titles are bad, but I am saying that more newer gamers, and even many growing older, that I'd imagine the ability to sit down and finish a game in a night or two, like one would a movie, wouldn't be a bad thing, especially if the gameplay never became exhausting and the game were cheap. As it stands now, I grow tired of lengthy games, even ones I enjoy a lot, halfway through. They're just too long. With a movie, it's a three-hour investment. With television, thirty minutes to an hour. Games can be longer, since they're interactive, but I'd be hesitant to say that a developer should look for a campaign over 10 hours.
In fact, there's a second type of casual gamer out there that's often overlooked, and that's the thirty-year old FPS player. They've embraced gaming just as much as anyone, but in a way that's seemingly traditional, and thus are grouped into the "hardcore" crowd without second thought. But truthfully, the formula Activision uses in it's Call of Duty line could easily be reproduced in other sectors: A loose but engaging story that unfolds as you play, relatively easy and uniform controls that can be picked up in a few minutes by most, a short campaign so that a few sessions will allow the gamer to absorb the full experience, and a multiplayer opportunity that is inviting to friends with similar tastes.
Yeah, I know, easier said than done, but truthfully? Take a look at New Super Mario Bros. Wii, and you'll find a lot in common. The problem is, truthfully, most "casual" or entry-level titles are a generation or two back on this process, and it shows. The formula for success is out there, and I think some developers are beginning to realize it.