What an excellent question! :-) And what an excellent test for our community. I think there are a few ways we distinguish ourselves, some of them very important.
For one, the answer rate of questions on the photography topic of Quora seems to be fairly low....there are hundreds of questions that have zero answers at all, while here at PhotoSE we have thousands of questions, and only a GRAND TOTAL of 5 questions unanswered. Of those 5 unanswered questions, 4 were asked within the last day, and could very well have answers by the time I am done writing this. Our effective answer rate is 100%! Quora has a far lower answer rate for photography with over 240 questions open and unanswered. Most questions here on PhotoSE get numerous answers, where as on Quora, there seems to be about 1-2 answers per answered question, assuming they get answers at all.
Second, Photography is simply an aspect of Quora, rather than its purpose. If you look at the "Ontology" of the Photography topic, it is a rather deeply nested "leaf" of a rather complex taxonomy of categories. Here at PhotoSE, photography is our entire purpose, and we dedicate all of the resources we have to offer to making PhotoSE the best place possible for photography-related Q&A, discussion, etc.
Third, while we may have only 6k registered members, we are a part of the StackExchange network. We have a potential immediate pool of users that is hundreds of thousands strong, and we are open to the world without the need for an invite. The nature of Quora is a single site that has a lot of categories, so management efforts are dedicated to "Quora". At StackExchange, we have specific, salaried people dedicated to servicing specific sites, such as our beloved CHAOS rep @Laura. She is responsible for maintaining a high level of quality across the board on PhotoSE, along with the hard working dedication of volunteers like @mattdm, @Imre, @rfusca, and many others, in addition to the current team of dedicated moderators.
Finally, the top answerer in the photography topic at Quora has offered 206 answers, and the runner up has only offered 120. The number of answers is the only real measure of a user at Quora. Our top users here on this site have offered around 500 answers each, and we have dozens of users who have offered more than 100 answers each. In addition to the number of answers provided, every single answer can be voted on, and the best answer can be "accepted" but the person asking the question. On Quora, voting exists, however they only seem to affect the order of display of answers on a question, and they are not a particularly visible aspect of each answer. Here on PhotoSE, votes are a key factor of our forum, are highly visible, and directly affect a users reputation. Reputation brings with it the ability for the membership to contribute to the community by helping to moderate, by editing content to keep it at a top-notch level of quality, etc. Beyond reputation, badges may be earned for different duties done and levels of service provided to the greater membership. All of these things factor into determining the quality of a member. When you get an answer from @MattGrum, @Itai, @mattdm, @JohnCavan, @StanRogers, @rfusca, and any one of our dozens of other involved, highly reputable members...you know your getting a quality answer that can be trusted for accuracy and completeness.
I think PhotoSE has a lot to offer over Quora when it comes to Photography. I think we are better suited in the long run to offer the photographic community the best place to come for top notch, respected, accurate, and complete answers to any of the questions related to photography, photographic processing, and photographic equipment. When you combine our Q&A forum with our chat and our blog, I think we have the potential to become far more than just a forum...
We are a community of like-minded photographers enjoying our passion to the fullest, and sharing it with the world.