Something I've been going on about, perhaps more at theRPGsite than here, was also touched on very strongly over at the Forge in this thread
Essentially, the nature of RPGs is such that you can't really nail down all the procedures and goals of play without turning them into board games, and the "wiggle room" that's left as a result turns out to be a big gaping breach. Depending on how you fill it, you can end up with a functional game in a wide range of styles, without "breaking the rules"--or you may have a crappy game altogether no matter how closely you try to follow the designer's supposedly "ready to play out of the box" intent.
Mike Holmes also hits on a point that I first saw suggested by Jim Henley: that in many cases, the way a game achieves "focus" isn't through the action of the rules (which are indeterminate), but symbolically and socially, basically by attracting "the right sort of people" and pushing away others. In the extreme scenario, it may be sufficient to simply apply an extrinsic label to a game: call it "narrativist", say, and then only "narrativists" will want to play it...so they'll all be playing together...so they'll have fun due to their shared values.