I still find a lot of value in Quora, but I guess directionally speaking, you're probably right that the mainstream audience is lowering the standard. Not sure if I heard you point that out, but you probably meant to say that it's possible to build a horizontal site that fits the needs of niche audiences - it's all just a question of product decisions. Twitter is a good example of how it can be done the right way: my stream is completely different from my wife's, and we both love using Twitter.
I recognize that user segmentation is easier to implement on a site that's focused on the social graph as opposed to on the interest graph, but I am not convinced that it's impossible. Ie, how about requiring social credit in a certain vertical before being able to ask a question in that same vertical? That credit could be earned with good answers to existing questions, or inherited from relevant 3rd party sites with applicable currency.
I recognize that user segmentation is easier to implement on a site that's focused on the social graph as opposed to on the interest graph, but I am not convinced that it's impossible. Ie, how about requiring social credit in a certain vertical before being able to ask a question in that same vertical? That credit could be earned with good answers to existing questions, or inherited from relevant 3rd party sites with applicable currency.