I’ve had my D750 for a little over a week now. I had a few days with it in NH, mostly inside, and a few days in Seattle / Redmond. I haven’t had enough time for a full evaluation, but here are some of my initial thoughts.

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• Size – The D750 is pretty compact for a camera with full frame sensor and a flash. It actually weighs 4 oz less than my 7D while barley increasing in physical dimensions. The result is a nearly identical feel to my 7D. Lighter than the 5D but heavier than the 6D (which has no flash and has more plastic). Very impressive.
• Auto ISO –  Canon’s Auto ISO will adjust the ISO so that it is at the lowest value that will still result in a correctly exposed image. If I’m using Aperture Priority mode the camera will adjust the shutter speed to compensate, but there is no way to customize it. Whatever shutter speed the camera chooses is what I get, even if its too slow to take a steady shot. Nikon allows me to not only define the minimum shutter speed, but can also automatically select it using 1/focal length rule (to reduce camera shake). I can tweak how closely it adheres to that rule; if I have a stabilized lens I can let it be a bit more lenient, if I don’t I can make it more strict. Nikon’s implementation also works with the manually set ISO and only kicks in if that ISO will not result in the correct exposure.  If I set the ISO to 800 Nikon’s system won’t kick in until that ISO isn’t stable. Canon’s will doesn’t allow a minimum ISO and will occasionally choose shutter speeds and ISO combinations that I don’t want.
• 3D Focus – This is one of the reasons that I bought the D750 and it has been impressive so far. I can choose a focus point over an object (faces work well) and the AF system will track it across the frame automatically. This isn’t your standard servo focus where you have to keep the focus point on the subject while it makes sure they are in focus – in 3D Focus mode the focus point will actually change as the subject moves across the viewfinder frame. It’s crazy to watch, but very cool. In addition to allowing me to get more well-focused action shots in a series it also speeds up my composition. Regardless of where the point is in the frame I just put it on my subject, hold the shutter button, and move the camera around. The focus point will shift as I reframe. It’s absolutely fantastic! The only Canons that have this feature are the crop sensor 7D Mark II and the $6,000 EOS 1D X Mark II. Canon’s implementation isn’t as good as Nikon’s either.
• Autofocus Speed – The Nikon hunts less when autofocusing and focuses much more quickly in dim light and on darker subjects than my Canon. The 5D Mark III is probably much better than my 7D but I don’t think it’s better than this.
• Autofocus Point Selection – I can choose from a group of 9, 21, or 51 focus points, one of the individual 51 points, auto, or 3D tracking. I can easily change the focus mode and the focus type (auto, single, or continuous) by holding the AF button on the left side of the camera and using the front and rear control dials. This is easier than on my Canon which required a button press plus a control dial and didn’t reflect the settings changes as visibly. I usually had to take my eye away from the viewfinder to verify my selections.
• Viewfinder – The viewfinder is brighter and has better graphics (including color icons) than my Canon. It’s brighter because it is full frame – a 5D or 6D would probably be equivalent – but the digital focusing screen is clearer and the viewfinder provides more information. Nikon also uses the viewfinder screen to display additional settings as the are scrolled through, such as autofocus mode. Canon only changes the tiny icons at the bottom of the viewfinder instead of displaying confirmation in the larger top area.
• Controls – I was worried about the number of buttons on the D750 and their placement. Nikons are a bit intimidating with buttons spread out across the entire camera body, but I’m learning to really like them. The DOF Preview button is much easier to reach than on my 7D despite being a small button, the AF Mode button is much easier to use, and the bracketing button is convenient. There are several buttons that do different things if you hold them and use a control dial. Buttons have more customization options too. Canon still has a better menu system for customization, but Nikon is catching up with its latest D5 and D500 interfaces.
• Rear Display – The display is a bit larger than on my 7D from a size perspective and it is articulated, but that’s not my favorite part of it. I really like the way that Nikon uses it. Whenever you change a setting, it is mirrored on the rear LCD. If I hit the ISO button to change ISO it will display it on the rear screen. So if I don’t have my camera up to my eye, I can see a nice large view of what I’m doing instead of looking at the tiny top LCD. Canon, are you listening? Also, I prefer the information displayed by Nikon’s Info button to Canon’s Quick Settings button. Nikon’s has more info and looks like a traditional LCD screen in terms of color and font use. Canon’s has always looked odd to me so I never used it.
• Top LCD Display – The D750 displays a few items on its top LCD even when it’s off. One of these things is the number of shots left on the SD card. It’s minor but useful. My Canon does not display anything when the camera is off. There is also an option to keep the display illuminated at all times, which I like to use in the evening. It’s even smart – the light turns off as soon as the camera’s standby timer is activated; it turns back on as soon as you depress the shutter button.
• Help Button – Most Nikons have a help button that displays info about the currently selected option on the various settings screens. These details are usually buried in the manual. Canon’s menus are more organized but they aren’t entirely self explanatory. This button is awesome.
• Autofocus Assist Lamp – Most Nikons have a dedicated lamp on the front of the body that lights to assist autofocus in low light. Canon pops the flash for this. Nikon’s system is both faster and less intrusive. Subjects aren’t as surprised by the small AF light as they are when the flash pops up and pulses at them several times. Canon may agree – the 7D Mark II has a dedicated autofocus assist lamp.
• Flash Commander Mode – This allows the pop up flash to control external flashes wirelessly. My 7D has this and I love it. Since the 5D Mark III and 6D don’t have a flash, I’d have to buy an additional accessory for this feature.
• Custom Continuous Low Speed – The D750 has two continuous shooting speeds – 3 fps and 6.5 fps. I can also customize the low speed between 1 and 6 fps. So if I want 4 fps or 2 fps for some reason, I can have it. Canon only has full speed and low speed.

Not Sure

• Ergonomics – While I’m getting use to the Nikon, my Canon feels better in my hands. Even though I can use Nikon’s rear wheel to scroll through images and menu items, nothing beats Canon’s control dial. It’s large, easy to find, and extremely quick. Canon’s menus are cleaner and better organized.
• Lenses – Nikon offers lenses at similar focal lengths to Canon, but they are often more expensive. The 70 – 200mm f/4 zoom lens that I bought cost $300 more than the Canon version and so far does not seem any better. Canon has more constant aperture zooms and also has a generous selection of refurbished lenses, which often go on sale. Lenses are an investment and I don’t plan to be buying and selling them over and over, but it makes the transition to Nikon more expensive than it would be the other way around.
• Battery Life – My first battery cycle on the D750 resulted in fewer shots than promised. Maybe it was because I spent a lot of time fiddling with settings and menus, but I only got 800 shots, a full 33% less than its rating. Maybe it will be better when I don’t fiddle with it so much.
• Continuous Shooting Buffer – The D750 can shoot continuously at 6.5 fps until its internal memory buffer fills. Once the buffer is full, the frame rate slows down as images are transferred to the memory card. Addressing this requires a card with write speeds above 220 MB per second, which the D750 doesn’t support (though they do exist). I can get about 14 RAW shots (2  seconds) at 6.5 fps before speeds drop down to about 2.0 fps. My 7D had a deeper buffer that could handle 25 frames. At 8 fps that gave me an extra second of shooting over my D750. Overall it’s not a huge deal, but it’s slower than I’d like due to the shallow buffer.
• Mode and Drive Lock Buttons – These must be depressed to change the camera mode and drive mode. I’ve never had issues with these dials slipping so I find these useless and annoying. The drive mode lock button is also very small and awkwardly shaped, making it a pain to push.
• Autofocus Point Display – My Canon always shows all of the available autofocus points as well as the selected one. The D750 only displays the selected point and some guide points at the edge of the focus area. I prefer Canon.
• Upper LCD Data & Organization – The top LCD displays less info than Canon (e.g. autofocus mode), however it makes up for some of that by displaying data in the viewfinder and the LCD screen.
• Shutter sound – My 7D sounds quieter and more crisp when the shutter fires. The D750 sounds abrupt and sluggish, almost like it is slapping shut.
• Cross-Type Focus Points – Cross-type focus points are sensitive to both horizontal and vertical lines; generally the more the better. My 7D has 19 focus points and all of them are cross-type. The D750 has 51 focus points and only 15 are cross-type (about 30% of the total). They require less light than Canon’s but there are fewer. That said, the other autofocus abilities of the camera may make up for its lack of cross-type points.
• No GPS – The 6D has built-in GPS, but that’s not enough for me to buy one.
• HDR Mode is JPEG Only – The D750 can create HDR images in-camera, but is limited to JPEG output only. The Canon 5D Mark III can output HDR RAW images. Luckily Lightroom can also create HDR images from RAW files.

Other

• Full frame Nikons allow you to set a 1.2x or 1.5x (DX) crop mode, which sounds cool but isn’t executed very well. All it does is draw a rectangle around the reduced image area; it doesn’t black out what won’t be in the image or enlarge what will. This makes it harder to frame the image and at the end of the day just crops out a bunch of data in-camera. Few people recommends this; it’s best to crop in post.
• Full Frame Nikons can use crop-specific lenses and will automatically set a 1.5x crop. This is nice for upgrading from a DX to a full frame body but it isn’t recommended otherwise. If you want to be crazy you can even turn off the crop with a DX lens and get the darkened corners of the smaller image circle. I like the flexibility it provides even if I don’t think I’d ever use it. Canon’s crop lenses are physically incompatible with its full-frame cameras.
• The D750, along with many other Nikons, have a bunch of silly in-camera filters that you can apply to your images – black and white, pencil drawing, fisheye, selective color, etc. I’m pretty sure I’ll never use these.
• The D750 is the first full frame SLR with an articulated screen. It’s neat but I haven’t used it a lot.
• The D750 includes several features that Canon does not offer at the same price range (or at all) such as wind noise filtering for the microphone, the ability to adjust the beep volume and its pitch, an interval timer, time lapse feature, auto distortion / vignette control, flicker reduction for florescent lighting (like the Canon 7D Mark II),  2-9 frame exposure bracketing, exposure fine tuning, and spot metering linked to the current autofocus point (only available on Canon’s $6000 1D X). These are various and sundry additions that are nice to have but don’t really factor into a brand choice decision for me.
• Nikon lenses still have mechanical aperture control. Canon’s are completely electric and internal. I’m worried about jamming something when I switch Nikon lenses. That said, Nikon bodies can still use lenses from the 60’s whereas Canon’s can only go back to the late 80’s.
• Lens-wise, Nikons lenses (at least the 105 – 120) are fatter and have tackier zoom rings. Sometimes the zoom and focus rings are reversed between the two. Both sets of lenses are similar in weight.