When someone posts something so arcane that virtually nobody "gets" it, no communication has taken place (unless you count the poster having communicated that he thinks he's smarter/cuter than everyone else on the site as "communication"). I don't see posting on an Internet forum as any different from writing an article.
This is my point. You're asking everyone to have the same lowest common denominator definition of what "common knowledge" is and is not. There is no such thing. Almost everyone has a different definition of "things everyone knows" and "things that are arcane". Because everyone has grown up in a different environment. People who watch a lot of TV have a different common frame of reference than people who don't own a TV. People who watch old movies have a different common frame of reference than those who've never seen one. People who have read Shakespeare have a different common frame of reference than those who read People Magazine. People who grew up on a farm have a different common frame of reference than people who grew up in a city high rise. I have a much different common frame of reference than my father. So what defines "common" from "arcane"?
The "What's the frequency Kenneth?" is a common frame of reference to likely a majority of APS forum members. Where do you draw the line for referencing it? At the 51% majority? 90%? Conversely, at what point do we not need to reference something? When only 10% of people don't get it? 1%? How do we know, given that we don't have a standardized frame of what is common knowledge and what is not? I've certainly seen posts from you Hawkmoon, talking about car stuff and I had no idea what it was. You're into cars, so what you wrote is part of what you don't even consciously consider might not be "common knowledge" to someone else. We all do that.
An Internet forum is not a publication. It's just a bunch of friends yacking in the backyard. If you don't know what something is, just say, "What the hell, is that?" the same way you would if you heard it in the backyard while drinking beer with everyone. There was a time (not that long ago)when a US education included a knowledge of basic Latin, so when authors inserted it (and still do to this day)in their writing, they didn't footnote phrases because people were supposed to know them as "common knowledge". If the reader didn't get it, they were supposed to look it up (in books, at the library, before the Internet).
To have to guess at what is "common knowledge" (or not) for every member on APS is simply impractical. Plus it kinda takes the fun out of posting.
