The D700 was "professional" level camera while Nikon themselves say on their website that the D750 is an "enthusiast" level camera. Now I definitely do not want to get into a huge debate as to what qualifies a person or a piece of gear as "pro", but I will say this: when you are using the camera all day, every day there are little things that matter. Things like build quality, the amount of detail shown on the top-panel LCD, sync speed (albeit just 1/3 of a stop), and having specific switches / buttons for each camera function.
I have not handled the D750 (or even the D610 for that matter), but I own two D700s and have used the D7000 extensively & D7100 a little. The feel of these cameras is completely different. When you put a heavy lens like the 70-200 on the front of a pro-level body, the magnesium alloy front panel makes a solid connection that you feel you can really trust. Personally, I never felt 100% confident putting such a heavy lens on a lower level camera - it always felt just a little fragile to me. That's not to say Nikon's lower level cameras are not well built - they are, but the pro bodies are just at a different level. We're obviously looking at different needs for different users here.
Similarly, I prefer having a button for each function, not having one button which does many different things depending on which mode or menu you're in. For most people this won't be a deal breaker, but for those whose camera rarely leaves their hands it can make all the difference. Again, little things.
I will not be selling my D700s to buy D750s because I do not feel like it's an upgrade. It's more of a side-step. I'd get some features that I know I'll like, but will lose many others that I use every single time I pick up the camera.
To answer your question about what a real D700 update would look like - I think most of us hoped for the D610 sensor in a D810 body.
I'm really not trying to talk down Nikon's enthusiast level cameras. They're fantastic, and for most people the D750 will be a massive upgrade. I'm just trying to explain what (I think) a lot of people are looking for in a "true D700 successor" camera and why the D750 is not it. (apologies for the long response)
Best regards.
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High speed sync can be a handy option from time to time, but it's not the same thing.
High speed sync strobes the flash so multiple pulses of light paint in strips of the frame as the shutter moves. But each time you increase your shutter speed by a stop, the maximum flash power drops by a stop. Also, it only works with the very latest flashes which support iTTL (not cheap).
On the other hand, with my D70 and an old SB24 I could shoot at f/2.8 and still overpower the midday sun because it happily synced at 1/2000 of a second with full-power flash.
Cheers!
