So, given that, what constitutes CommonContext? Television and media nowadays, I'd surmise. Basic education, situational circumstances, the human condition...
Anything, really. People who have to talk to each other will find common ground. Even if it reduces to, "So, nice weather we've had lately, eh?"
It's important to remember this when making a community (common ~ community). You need to provide some central context to revolve around or else the "community" will seem fractured and chaotic. -- SunirShah
See also WhatIsContext
One important factor is the degree of chosen context. For instance, at work I know a few ScienceFiction enthusiasts, but most of them have different tastes in SF books. If I go to a written science-fiction convention, however, nearly everyone will like SF, and it is easier to find people sharing my particular interests. My workplace is a partially-chosen context (people choose their line of work, but rarely choose specific locations), while the convention-visitors have a very free choice whether to visit.
In the past, one's community was defined by geography, but communities are increasingly defined by shared interests. (I know far more about SunirShah than I know about my neighbors.) At least in the United States, many people take it for granted that they may move across the country to follow their interests or career.
Within the Internet, the nature of websites create a new kind of geography. For instance, SlashDot is a community with rules and customs, and even a kind of social class system (through moderation and "karma"). The content of SlashDot is controlled largely by a small team of editors who choose top-level stories for publication. If a small group disagrees with the SlashDot policies, they can do very little except argue their position.
The C2 wiki also occupies a kind of "space". While one is technically free to make major alterations, one is socially very constrained within most wikis, and in some respects more limited than sites like SlashDot. Within SlashDot, people that disagree can each argue their ideas in separate reply-articles. In wikis disagreements often concern entire pages, and attempt to force a consensus view.
SlashDot is paradoxically more open to "grafitti-like" content (like "FIRST POST!!!" or hot grits, etc.) than the C2 wiki. On SlashDot, such contributions are accepted, and quickly moderated downward. On the C2 wiki, such content is quickly removed (even sometimes when it is marginally valuable). Several people have felt "run out of town" from the C2 wiki. A few people will argue that any content is allowed, but several influential people see the C2 wiki as a software development site (with occasional approved diversions).
In some ways, one has only a limited choice about one's online "neighborhood". Content is tied to particular systems--I am not generally free to copy an essay from SlashDot or C2. Tools like the SlashDot moderation system can help segregate people by their interests, but one is still limited by other people's choices.
ViewPoint will attempt to remove many of the limits in existing online communities. Content will not be tied to a particular page or site, and filtering/referral systems can be based on any method a user desires. If one wants a moderated/edited "journal", or a Stone Society, or a collective "council", or even an open wiki-like view, then ViewPoint will try to provide support for the view. More on this later. --CliffordAdams
Another good example of CommonContext: ScienceFiction/SpeculativeFiction. SF often has to build a common context between the characters in the story and TheAudience or else the relationship between the two would break in confusion. Much of SF spends its time explaining what is normal and ignored to characters in the story, which is complete cruft in "normal" literature. If the characters don't care, neither should the audience, but you just have to spend time explaining how toilets work to set up enough common context to set up the plumbing jokes. -- SunirShah
This idea of CommonContext includes such things as what CulturalDimensions you may have versus that of your audience. Western culture of course provides enough common ground for most people from the United States and Western Europe to converse reasonably (provided they're speaking the same language of course). In contrast, there's Asian cultures (cf. XiongChagnian? sp?) where a different context may preclude such things as RealNames, or bounds of politeness. (Chinese people expect to be offered a gift multiple times and refuse the gift at least once, to be seen as not-greedy, before accepting it, for example - which can lead to consternation when the Chinese hostess offers a Western person a gift which is happily and/or greedily accepted.) -- NatalieBrown