I don't think that unambiguously set boundaries are necessary given a set level of expectation prior to beginning. Human beings are able to deduce the boundaries of their environment without explicitly being informed of what they are to greater degrees as they mature. The reason humanity does not need to create rules to govern social interaction is because it is generally a given that we know and will abide by those unspoken regulations. When somebody violates these guidelines, it makes people uncomfortable and pushes them to assume that there is something wrong with that person for that very reason. Boundaries are best when self-set because when operating on a high level, you know what the logical result of a set of actions will be. Otherwise, you end up with a massive rule structure because there's no way to cover every possible case when people will abide by the letter of the law and not the spirit of it. So long as there is reasonable trust that others will be both mature enough to understand these implicit boundaries as well as reasonable enough to respect them, there is little reason to implement a particularly rigid structure for boundaries.
Of course, there is the distinct possibility that somebody driving the story wishes to go about it a certain way, but in the end, that also limits the creativity and purpose of this site entirely—introducing other individuals also introduces randomness and unique paths. The greater the restrictions on the possible paths, the farther it drifts from a collaboratively driven work and the closer it approaches a work by a singular author who may have had several ghost writers assist him in the creation process. While this is not necessarily an inherently negative way of going about things, I do believe that it goes against the spirit of these games or stories in the first place. While somebody may prefer a "healthy medium," the governing structure and restrictions on a story (or lack thereof) will cause the medium in its entirety to drift toward one of those two extremes. While I personally prefer to choose an extreme and create a foundation on top of it, I can see why others may be inclined as to choose another route. I feel as they may be limiting their own enjoyment in the end by doing so (though perhaps not; that is not for me to say), but that is another discussion entirely and outside the scope of this thread.