I love my D750 as well - well enough that I bought a second body as a spare. My last-previous camera was a D600 (almost exactly the same), before that, a D7000 (my D series lenses worked on that). The major thing you get when moving to FF is that all your lenses return to normal (ex-film guy here) - and you get a lot more dynamic range - to the point that I stopped bothering with stacking exposures in less than extreme situations (e.g. shooting from inside a dark cave into bright sunlit spires).

The camera is fabulously adjustable. If you don't already have it, get a copy of Thom Hogan's alternate manual for it. As well, Steve Perry has a couple of good ebooks on the minutia of focusing and exposure modes. (Zone system guy here; I'm mostly using it as a match-needle camera with spot exposure mode).

Programming that top video button as ISO, is mostly useful when the camera's on tripod, and you're looking down at it.

Also, I love the programmability of the buttons. I've set the front Fn button for focus. BBF on a back button doesn't work for me as I'm left-eyed, so my face is in the way. There's plenty out on the 'net about BBF, but if you're unfamiliar, it's super useful, particularly for subjects that might start moving.

On top of all that, the D750 is a low-light monster (it was dark enough that navigating back through the woods was challenging).