(6) How is Physics Overflow different from other physics sites?
As of today, 25/10/23/2/2014, Physics Overflow is clearly different from any other Physics Q&A site.
Physics Stack Exchange
Physics Stack Exchange is a site for general physics, including popular physics, low-level physics, etc. The policies and politics of Physics Stack Exchange is also drastically different. E.g. Physics Stack Exchange encourages niceness, while Physics Overflow was born with the intention of encoruaging frankness, and not having to be "polite-ically correct".
Quora is more for popular-level questions, as opposed to technical questions. It is also very broad-scope.
MathsOverflow is primarily a research-level mathematics site, even though mathematical physics is allowed.
TheoreticalPhysics.SE was a site only for research-level theoretical physics. PhysicsOverflow's Q&A section is some sort of a revival of TheoreticalPhysics.SE, but with a much broader scope. We accept not only Theoretical Physics, but also Experimental Physics, Astronomy, and Phenomenology. Our scope is also more broad, since our scope is not exclusively research-level, but graduate-level and above.
All the questions from Theoretical Physics Stack Exchange have been imported into PhysicsOverflow, and all the graduate-level ones on Physics Stack Exchange will soon be.
PhysicsOverflow.com was a SE-1.0 site which was later taken over by Physics Stack Exchange. It was mostly populated with sub-graduate-level questions. It is not affilated in any way with PhysicsOverflow.