I would love feedback on my startup, Lawdingo.com (YC W13), as I believe it addresses many of these issues. We get independent lawyers to be transparent about their prices, we solicit community reviews, and we make lawyers available instantly. I'd humbly recommend people try it out for their startup legal needs. It's free.
As a lawyer I am always interested in sites that want to try to change the legal profession in some way. My thoughts: Do you vet the attorneys in some way, ie, require a bar license number to become a lawyer on the site? Usually on the consumer side these sites just turn into a race for the bottom price, with no real regard for the quality of the representation that will be given. Frankly that is a problem for all of these types of sites, it should be a concern for you in terms of attracting quality lawyers to the site, a quality lawyer with a quality practice will not necessarily want to get into a position of competing on lower and lower prices.