Why does Fred Wilson think that a lot of vertical Q&A sites like StackOverflow will perform better than horizontal Q&A sites like Quora?

Remember vertical search engines?  They were all the rage.  Oh, yeah, I still Google that.

Fred is partially right, but mostly wrong.  It is easier to create a vibrant community around a narrow topic, but substantially less valuable:   Vertical Q & A sites can be moderately successful but are unlikely to create billions of dollars of value or fundamentally transform the Internet.

Why?
It is easier to build for the reasons Ben identifies: Dual-sided markets require hand-crafting to ensure equilibrium between questions and expertise. Trying to balance these across multiple verticals simultaneously is very difficult.  One can argue that this was a key mistake we made at LinkedIn Answers, although I am not persuaded it is the true cause.

The problem to the vertical approach is that it produces subpar engagement and retention.  It is extremely challenging to persuade users to revisit your site on a daily or even weekly basis if it does not serve a broad purpose. 

To wit, Craigslist is insulated from most e-commerce assaults because of its combination of personal ads, job listings and housing options.  The personals and sex ads account for a majority of the page views and retention even though the other sections drive their revenues.  Likewise, the general purpose Amazon is now on the precipice of eating up much of e-commerce:  http://www.businessinsider.com/c....

Back to Q&A:  In a vertical site, you would never generate some of the most compelling Quora answers which are often very far removed from startups, venture capital and engineering, although mostly written by those who are professionally dedicated to those fields. (See the most popular answers for a sample of some very random topics:  Quora Usage Statistics and Analytics: What answers on Quora have 40 or more upvotes? )  Quora would feel like a professional obligation, not something to look forward to.  Most people procrastinate chores.