Everyone has been in this situation at least once in their life. Your sitting at a table with a group of people having a general discussion on some random topic. Then a friend in the group makes a bad joke referring to something only you and they know about. You both laugh, others looks bemused, and as is the case with any "in joke" its all the more amusing cause not everyone "gets it".
Broad appeal marketing works much the same way only (strangely enough) in reverse. As an advertiser, quite often a campaign idea will be scrapped due to its alienation of certain demographics. If we are putting together a campaign for say the Toyota Corolla, we don't want to use images and language that will alienate the over 60 crowd because as this might result in lost sales, but at the same time we don't want the car to be branded as a "grandma car" cause this might lose sales in the younger age group. So what do we do? We choose THE MOST middle class, middle age bracket people and dress them in non biased fashion so we can get the broadest appeal possible. And what ends up happening? Everyone at the table "gets it" so no one finds it all that funny.
As marketers are finding out, reaching the Gen X/Y markets is far tougher than it was with the baby boomers. These new customers are infinitely more fragmented and are constantly narrowing their own demographic. You just need to look at the shear number of sub genre's in popular music to get a feel for how things are changing, and they (the Gen X/Yers) like it this way. The less people there are in their genre the more exclusive it is and the more individual they feel.
So if your potential customers are deliberately fragmenting themselves how can you expect broad appeal advertising strategies to connect with anyone? You can't.
I look forward to seeing a main stream brand use a term like "L337" in a television campaign. Sure there will be some people that wont have a clue what it means, but for the viewer that DOES know what it means, it will be like the ad is speaking directly to them.