It has great ergonomics, in-body stabilisation, faster than A7 series Autofocus (but not so sophisticated as Nikon's), faster burst shooting, extremely useful articulated screen. And direct (w/o adapters) support for great lens like AF Zeiss primes and zooms (different to non-af versions), especially 2/24 Distagon, 1.4/50 Planar , 1.8/135 Sonnar and some others and some interesting legacy Minolta's (4/200 Macro, 1x-3x Macro Zoom, 2/28, 2/35, 2/100, 100 Softfocus, 135 STF and so on).
In other cases it's modern camera but could be better - personally i don't like a little laggy operation (due EVF startup time) and not-so-good video quality - still better than long-time industry standard 5DMkII (in 2014 we have many wonderful options).
In your case it can be replacement for your D800 for long lenses IF Sony A system can cover your needs - a99 is more handy for lens like 70-200 and 70-400 (both excellent) than a7 with adapters.