IQ/Noise. This is the biggest issue. But living with 8.5MP for 7 years showed me, that the quality of an Image is not measured in ISO, Noise or dynamic range.Sure when lighting is right, technology doesn't matter. But outside of the studio it rarely is right
Having said this, I think neither noise nor IQ really differs between crop and FF, you basically just gain about 2 stops. Even DOF would be more shallow when shot with an APS-C and the 50mm 1.2 than in most FF-shots. In the end it's rather about which lenses one uses, than about sensor size.
As for the smaller viewfinder: This is really a plus for the FF's. But it doesn't have to be that small, the 7d viewfinder is only 10% smaller than FF viewfinders.