I was sure it would be a Nikon for you, Rod, and the D750 looks like an excellent camera. Once you have committed to a particular premier brand of (D)SLR and have amassed a collection of lenses for that brand, it is virtually impossible to change. I am a Canon devotee as you know, and I am unlikely to change my allegiance, having acquired a lot of Canon lenses over the years (not because I think Canon is any better than Nikon).

I am not sure how far the backwards compatibility of the Nikon goes, but the current Canons will work with all Canon EF lenses going back to 1987, not bad in my estimation. Unfortunately, many of the older models of independent lenses (Sigma, Tamron, Tokina etc) cannot always be relied on to be compatible with the newer Canon bodies. Hopefully Nikon may be better in that respect.

The good low-light performance of full-frame cameras is a well-known feature, presumably because the area of the photo-sensors is so much bigger. More photons hitting each pixel site must be a good thing. I look forward to seeing the results from your new toy, but I am sure you will still use your crop-sensor Nikon as the mood takes you. 

Yes indeed, I do have a few old lenses bought for my FE and FM bought in the 70s and 80s. They are still very sharp and despite the lack of automation are still a delight. Back then, I never graduated to auto focus and all my lenses were primes. Now, zoom quality is greatly improved and my old peepers quite like auto focus.  

In my experience, the autofocus on DSLRs (and on those few film SLRs equipped with TTL autofocus) will generally give better focus accuracy than manual focussing and is almost instantaneous (certainly faster than I can do manually). Except for a few special situations such as macro or portrait photography, I rarely use manual focussing now. Some of it is because of the difficulty I have of getting a good view in the viewfinder, although a full-frame camera has a bigger and brighter viewfinder than a crop sensor camera. As you imply, Rod, manual focus is a challenge to the old peepers. Of course there is the option with most DSLRs of checking focus in live view with the rear viewing screen, but this can be slow and the screen is difficult to see in bright light. 

Back in the old days, without  autofocus, we could only rely on squinting at the focussing screen to achieve proper focus manually. To get the best results, I remember that most SLRs offered interchangeble focus screens so you could select the optimum type of focussing screen for the job. I think this is still the case for pro and semi-pro DSLRs, but I don't hear of people bothering to change focussing screens, since autofocus is so good now.

I did change the focus screen on my Nikon FE. Now with the clearer viewfinders and full frame sensors, I have not even considered doing this. 

Recently I have been taking some macro shots and the quality out of my cropped sensor Nikon D300 have pleased me, especially as they were shot through the cheapest macro (micro) lens that Nikon make. Not yet time to put my D300 into mothballs.