Chris Anderson has a nice riff on the role of the filter vs. the friend (that is, the middleman/editor vs. the recommender/neighbor) in the post-scarcity, mass-of-niches era of media and products.
In the existing Short Tail markets, where distribution is expensive and shelf space is at a premium, the supply side of the market has to be exceedingly discriminating in what it lets through. These producers, retailers and marketers have made a science of trying to guess what people will want, to improve their odds of picking winners. They don’t always guess right–there are surely as many things that deserved to make it market but were overlooked as there are things that made it to market and then flopped–but the survivors get a reputation for some sort of mystical insight into the consumer psyche.
But in Long Tail markets, where distribution is cheap and shelf space is plentiful, the safe bet is to assume that everything is eventually going to be available. The role of filter then shifts from gatekeeper to advisor. Rather than predicting taste, post-filters such as Google measure it. Rather than lumping consumer into pre-determined demographic and psychographic categories, post-filters such as Amazon’s custom recommendations treat them like individuals who reveal their likes and dislikes through their behavior. Rather than keeping things off the market, post-filters such as MP3 blogs create a markets for things that are already available by stimulating demand for them.
It can also be expressed as first-person vs. third-person markets.