Today I made the big switch, ditched the D800 for the D750. Kept the D700, since they wouldn't even give me $500 for it. It's worth more to me than that, in terms of usefulness as a reserve camera.
The first things I noticed, while playing with the in store demo, were the grip, lighter weight, and the snappier auto focus. Testing the camera in the field later in the day I noticed this more and more, particularly with moving subjects. Even the 70-300mm F4.5-5.6G VR felt like a new lens, the AF is just that much snappier than the D800. The Expeed4 really makes the entire camera just feel a little bit more responsive, in terms of menu moment, liveview focusing, and so on. I haven't really noticed any ill effects from the slightly tighter arrangement of the AF points, but I'm sure I will later on when I do some portraits. Oh, speaking of AF, I no longer need AF fine tune with the Sigma 50mm F1.4 Art.
I haven't played with features like WIFI or anything like that, but all that will come in time.