Lowest Common Denominator is used to describe the state of affairs where the quality of information content in terms of SignalToNoiseRatio converges towards zero as the size of a community increases. This AntiPattern seems to have particular applicability to virtual communities as its effects have been quite evident in the BBS culture of the 80's, and even more so on the Internet, perhaps due to the fact that these are both what we might consider OpenSocietys.
Contrast this with academic society. It seems that the same does not hold true due to that society's closed nature. In order to be admitted, there is a formal screening process intended to minimize the entry of individuals who would lower the quality of the infomation.
I'm not sure how the notion of information coherence fits into this, but have the feeling it's a separate-but-related issue.
As an illustration, consider the average mental health of innocuous newsgroups like news:sci.math or news:alt.philosophy before and then after the AolizationOfTheInternet. Today these groups attract the most demented crud and drive away the highest common denominators.