Just thought I'd post my initial thoughts on the D750. I've had mine for a couple of days and so far I'm pretty impressed although it's taken a little while to impress me.
I won't go into the unboxing, if you've had Nikon, they're all the same.
Having come from the D700, the D750 is noticeably smaller and lighter and my initial thoughts were that the construction isn't as robust. I think it's more of the issue with the weight, size and the quieter shutter rather than construction so it could just have been my impression rather than a formal evaluation. The battery grip only takes 6 AA's which might explain the lack of higher frame rate with it.
It takes a while to get used to autofocus system which is substantially different. Initially I thought it was crap but it came down to understanding how to use it and the good old RTFM. Once I understood what and how to use it, its actually a phenomenal piece of engineering. The low light focus is incredible. I think new DSLR users or those coming from base level models might battle with it and may end up sticking it on auto all the time because it can be confusing. I did test the auto function and it's really not the bad all things consider. I think most consumer users will like it but I think most of the more advanced users would have limited use for it.
The new highlight metering is awesome and is quickly becoming one of my favourites.
ISO blows my D700 away which is not surprising given it's two generations more advanced and about 5 years newer which in the tech world is a long time.
I'll start off with likes
Wifi - release your shutter with a mobile phone, awesome!
New autofocus system - insane low light focus capability but takes some getting used to
Quieter shutter (takes some getting used to)
Video - Don't think I'll use it much but it's good to have it. having DOF on video is awesome although you can hear the VR working in the video
Tilt screen - I put it under likes because there are some occasions where it's nice. Shots where you're holding it high or low, but I reckon I will use it 1% of the time because I don't do much video
Removal of dedicated AF button and metering switch
Smaller size (I have big hands although others may like it)
Doesn't have the multiple custom settings options like the D700
Consumer features like scenes although I could use them for my wife when I hand the cam to her.
Here is an ISO12800 shot, good for Facebook and similar applications, I applied a little noise reduction in lightroom but I haven't done anything other than a quick and nasty.
A couple more shots testing the autofocus system along with some testing the auto mode in the focus system