One of the promises of full frame is increased image quality compared to APS-C sensors, and the other is better depth of field control.
I am quite happy with the image quality from the A7, despite the lossy compression of the RAW files that Sony decided to apply to RAW. I think this is inexcusable. The reason to shoot RAW is to extract as much details and obtain maximum image quality. Lossy compression of the RAW files defeats that purpose. Sony should at least provide an option to choose lossy and lossless compression on RAW.
Other than the lossy compression issue on RAW files, I have no other qualms with image quality from the A7. The 24 MP full frame sensor is capable of capturing an amazing amount of details, provided the lens has good enough resolving power. So far, I have used the Pentax-M 20mm f4, ultra wide angle, the Canon FD 55mm f1.2 S.S.C, and Canon nFD 50mm f1.2, FE 28-70mm kit lens and of course the Zeiss Sonnar FE 35mm f2.8 ZA, and with the exception of the kit zoom, they all perform really well on the A7.
Depth of field control is excellent on full frame. With an f1.2 lens, you can get paper thin depth of field if required but is harder or impossible to achieve with smaller sensors. I truly feel liberated when using my legacy lenses; they are finally being used as the design intended. This is the major reason I bought a full frame camera.
Man at work - Sony A7 & Makinon 300mm f5.6 Mirror Lens. Click for larger.